Old fire truck

The only 22.5" tire with a 43" diameter I can find is a 12x22.5 that REQUIRES a 9" rim, and would take a wider spoke (front to back on the spoke web) than your hub would have. Likely need a 4 1/2" spacer band or wider.
Measure the spokes on your bus and your L/S, measure from where they start to taper up at the inboard end to the end of the spoke where the stud is, and compare.
Post the tire mfg web spec's please! or the mfg and tire size.
The diff has almost nothing to do with it, it is the frame springs and hub will determine what will fit.
Post the width of the spacer band on the bus and on the L/S.
DO NOT think you can interchange spacer bands between different hubs, Wedges and bands are determined by the hub and can not be interchanged.
The wider the rim, the more the offset change, so the closer it will be to the inside frame/springs.
Rims over 7.5" for 20" rim or 8.25" for 22.5" rims require special wedges to prevent breaking of the studs. The cast spoke must have the clearance for the longer wedges. (on the front hubs).
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Excellent, that info I can use thank you! I'll try to get some pics up this weekend. As far as the tire. https://www.tirerecappers.com/tires/sem ... tire-copy/

I would question their 43" claim, that may be static diameter but what really counts is RPM (revs per mile) The biggest off road tread, 11x 22.5" tire I can find from a tire mfg has a 42" dia and a 31/32" tread depth. They only claim 26/32" So either they are wrong, adding close to an inch of rubber between the casing and tread, or something doesn't add up.
Most all on hiway tread 11x 22.5" rubber is around 41" or 500 RPM. It can vary a few rev's per mile but that doesn't change the top speed much if at all.
SO are you saying your going to run caps on the steer as well? Better be **** sure of the quality!
Look at Michelin, Goodyear, Hankook or other Mfg website for RPM data, and tread design variations.
I run caps, but only my casings, where I know the history, and age of the casing and run only hiway tread (which runs cooler) because heat is the death of tires. A tire failure on the steer axle can have bad results, I wouldn't use unknown caps in that position.

Good advice thank you sir! I have done a lot of checking and reading on caps and have run 5-6 sets. Heaviest was some 19.5 14ply from treadright tire. Used those on a heavy pickup truck hauling wood up to about 23,000lbs. They were an aggressive MT style that wore very well. The estimated finished weight of this vehicle should come in under 20k. Also as I said early on I will not run on the interstate or over 65 mph. And 65 will be the exception to 50-55 driving on 2 lane roads with daily travel of about 200-300 miles. I think with lower speeds combined with being well under the tire weight capacity should help keep them cool and I do feel like a well constructed inspected retread would be safe on the steer. I'm a bit away from retirement so I'd like to get this done in the next 1-2 years max. Then I'll run it with a flatbed and use for wood hauling and heavy towing to work out any kinks that may crop up before hitting the road with the wife.

Looking at ply and weight ratings those 22.5's should be good for 6300lbs+ run as duals and roughly 6900 lbs as singles. If I round down to 6300 lbs X6 that's almost 38,000 pounds. Is that how tire weight ratings are figured? Makes sense if it is. That also reaffirms my thinking that I'll be way under weight for the tire ratings. That should help from a safety standpoint I would think?

Is Ethan back?

I don't know where you get your number from, a std 14 ply (LR G) in 11x 22.5 is 6175 per tire SINGLE, less dual. A load range H is 6610lbs SINGLE less in dual. I have never seen a 11 x 22.5 in a higher load range than 16 ply (LR H) anywhere on any tire mfg catalog. MAY be some NHS forklift casings But they will be branded NHS which means not for hiway service and would be illegal on a truck.
Tread design has a lot to do with how much heat and rolling resistance a tire has, Lugs are going to put more heat into the tire and have a higher rolling resistance than hiway tread, and it does that regardless of load, but more load will put more heat in faster.
It would seam that all the numbers you are getting from that tire site are inflated. It said right on there that they will not select for load range.
A 295/80 22.5 has a higher load capacity than an 11x 22.5 in the same diameter and slightly wider tire, but needs a 9" rim to use that capacity.
I've had one cap failure in the last decade, the tire shop refunded my money for the capping AND paid for a NEW tire to replace their failed cap, would the business you are dealing with do that?
The 11x 22.5 tires range from 496 Revs per mile (for off road lug) to around 503 ( for a shallow trailer) with the general speedo calibration using 500 RPM to calibrate. Some tire treads have a 50,or 55 MPH limit because of the heating problem.
Even if you got a tire with a low RPM, that only applies when the tire is new, and increase over time.
I think the best you are going to get is 63 MPH out of that set-up, which is likely all it should be pushed too anyway.

Those numbers were just generic numbers gathered from various web sites that posted tire specs. I didn't see numbers for
that specific tire in the link. With the numbers that you provided I'll still be way under the rating for the tires. I was just pointing out that there will be a good margin of safety in that respect. As far as speed goes 63 mph will be way more than enough for my needs. I realize there is always a compromise when you're designing a dual purpose vehicle. You've been a wealth of knowledge, thank you! I don't have tons of experience in medium duty truck stuff. Most that do only want to help if you're a fleet manager or have a bunch of trucks. So finding someone to bounce ideas to has been hard. I'm trying to get this built as inexpensive as possible and still keep it safe and reliable. I'm just under 5k so far and have a long ways to go.

Here's a picture of the truck.

Front and rear diff with a look at clearance.