Converting a 1973 1310 from gas to diesel

CB89 has not included any information on the service truck. First off, that 6.9 was a poor choice for the job because it is underpowered.
I am very familiar with service truck use. The company I worked for had an F450 with a Ford V10. This truck carried a compressor system along with 200 liters of fuel and 100 liters of 15/40 oil. There was two 8D batteries with cable connections front and rear for boosting. At the back was a small crane. The front push bumper looked heavy. The F450 was 4X4. This service truck weighed 15,000 pounds.
The truck did city road calls, in traffic, lots of yard pushing and towing.
A service truck engine is often idling at a call out. When this is all combined, the F450 consumed gasoline at the rate of 1 liter / kilometer. About 3 mpg.
The replacement Ram with the same size chassis and service box and same duties
Gets 12 miles per gallon. The Ram has a Cummins engine. Both trucks have automatic transmission.
Anything a gasoline engine can do, and Diesel engine can do as well or better and more economically.

Well, we have nothing to compare, as the OP didn't state the weight and size of what was being towed.
In my case, I would expect the 6.9 to give slightly better mileage than a gasoline engine of similar displacement and/or power.
With only the picture posted to go on, I would not expect the 6.9 to get 20 mpg in that application, regardless of weight.
The Cummins would give better mileage in my opinion, but even it would not approach 20 MPG in that service.
The service body on the truck I drove had less of a frontage area than a typical slide in camper, and only slightly more than a cap or "topper". In my case it weighed between 10-12 k depending on what I had onboard at the time.
At higher speeds, wind resistance plays a much bigger roll in fuel useage than does weight.
I also briefly drove a Powerstroke 7.3 DI turbo with a 5 speed and the mileage was only slightly better, in the 7's.
To try and justify a engine swap on economic grounds is a hard sell. To ever expect mileage of a vehicle of that type, size used in towing of any kind, to approach 20 MPG is wishful.

My 1953 is not a good comparison to the OP truck. My "R120" weighed 5577# after the Diesel 4X4 conversion. The on-line specs suggest the OEM weight was 3550#. The SD220 never got 10 mpg. the "R" with the 6.9 V8 Diesel regularly got 15+ mpg in town and in overdrive 20+ was not impossible on the highway. If the 6.9 was pushed it would consume fuel like a loaded semi and make smoke.
I had a large single exhaust, with a large quiet muffler, on the 6.9. I dropped it off at the muffler shop that did the original install and they changed to dual exhaust pipes. The sound changed to crap.

These are only fun numbers and nothing more. I still think that 6.9 had one of the best engine sounds. But then I have disqualify myself, I like the sound of a Detroit two stroke. I don't want one, but,...……..
Maybe the OP should consider a Detroit four cylinder. Varoom, Varoom. It will fit.
https://youtu.be/vQXZSgDN9uA
https://youtu.be/RZ7jMl6-EHE

My K's both turn about 10 MPG, One is 7K empty weight and the other is 10,500 empty. Long road trips they get close to 11, lots of intown cold starts its down in the single digits. One caveat "long road trips" are made at speeds of 55 or less.

My truck as pictured weighs right at 7,455 pounds unloaded with only about 5 gallons of gas. This weight is from a state certified and inspected scale.
There is nothing on this truck that is not heavy duty.
392 C.I. V-8, T-34 5 speed O.D. Transmission, New Process 205 Transfer Case (divorced), Spicer 70R Powerlok Axles (Front FA-52 4,000 pound, Rear RA-63 7,500 pound) 4.10:1 gears, Rear Springs are 7,500 pound, Front Springs are 3,200 pound.
I think I will go back to single rear tires as I will not be hauling and towing the weights I use too, so shouldn't need the duals.
If I do make the change to a diesel it will be to the 7.3L PowerStroke (T-444E). I do like the 5.9L Cummins and it very well might be a better engine, but it is not an International (basically) engine.

That said, can we get back to the original post questions?

That said, can we get back to the original post questions?

Well, you were going on about why you needed to change to a diesel, and we addressed that. If in fact you question is: I am going to put a 7.3 in my truck (regardless of reason) has anybody done that? It should have been stated as such.
I doubt anyone here has 1st hand experience on that exact swap. Nikki had done one on an earlier model and I have a fair bit with the engine in general.
1st thing to address is if the current drivetrain will bolt up to the engine? From what I remember Ford uses a small adaptor between the engine and the bell they use. I don't think the 7.3 block bolt pattern is identical to the 392, so that will need to be verified and dealt with.
A 7.3 is bigger and heavier then the engine it will replace, and from what I remember of the engine bay of 70's light line, there isn't a whole lot to spare.
Hooking up the ancillaries is going to be the least of you problems, and if you are uncomfortable attacking that, I would caution that getting the engine in, and serviceable is a bigger job.
Since you are talking about a mechanical 7.3 not a DI powerstroke, then all that is required is a little engineering of linkages.
The 444 was used in a lot of medium trucks by IHC, so finding a bell that would mate to the current trans might not be that hard, but adapting to the chassie is going to take some work. Clutch linkage comes to mind here.
Most medium and heavy engines are supported at the front and at the rear bell and not from the side like most light engines are. At some point IHC used side mounts on the SV series, I don't remember if the model year you have is one of them. If so it may require adapting the Ford mounts to the frame. The IHC bell will likely be set-up for crossmember mount at the bell.
I have to admit I am a bit confused as you say this is your primary tow and hauling vehicle and later you say you are going to pull the dual rears as you are not towing? Which is it?
All in all it is going to take a lot of work, a lot of fabrication, and a lot of searching to find parts that will mate together.
Only you can decide if it is worth the work, time and money. You can buy a lot of gas for what it is going to cost.

Re-reading your 1st post, I see that you are not planning to do the work yourself, I would say that alone will make the job cost prohibitive.
That truck was likely spec'd for a utility maintaince vehicle, and fuel economy took a back seat to other concerns.
Unless you can find a shop that has done many of the exact conversions, I doubt you'll even find one that can give a reliable estimate, just too many unknowns. The best you could hope for is time and material, which is basically a blank check. What will you do if it is 1/3rd or 1/2 way done and already exceeds your budget?

What was being hauled and towed put a combined weight around 18,000 pounds. What will be hauled and towed from now on should stay around 14,000 pounds or less, remember the truck itself will account for 8,000 pounds with fuel.
The mileage information is great, but what needs to be modified on an R to make the motor fit doesn't seem to apply since the bodies and frames are very different.
Also, as I am totally discussed listening to any diesel truck which has an exhaust system modified to make more noise, which engine sounds better is not important.

Given the size, empty weight of the truck, windage and the expected towing weight, At best I think you might gain 1.5 MPG. That would take a lifetime to earn back the money it took to convert.
Again, my advice would be to put the money in fuel tank and enjoy the truck as is. To do otherwise, I expect to see a pile of parts that can't move on its own and all funds exhausted.
I know that is not what you want to hear, but it is as realistic as I can make it.
This is even true if you were planning to do all the work yourself, and you had the equipment, tooling and experience to make anything needed in the swap.

These are the only pictures I have of the engine area.
I hadn't thought about using the current transmission, was thinking that I would find a donor truck for the engine and transmission, but it's something to consider.
I would (I think I have the ability) do the work myself but I do not have access to a shop with hoists/cranes or the machining equipment to fabricate items like motor mounts and cross members. It would depend on how complicated the engine and transmission linkage would be as to whether I could do those. I don't think I would have a problem with wiring gauges and other such things.

It would appear that my guesstimate of $12,000.00 to the conversion done should at least be doubled if not tripled. And finding a shop that would do the work would be difficult at best.