Old fire truck

I go back to what I said several pages ago, look at the gears for a makers mark and a part number that can send you on a search of what is available in gearing, it may be the answer is nothing, but it may not. Until you know who hobbed the gears, you don't know if there are other choices. Where the front gearset needed to match what the OEM supplied to the rear, it seams unlikely that Napco made their own carrier and gearset, but more likely that they made the housing to accept another mfgs already in production gear sets and carrier.
IHC made a few rear axles that had very limited ratio selection, so it may be, that there is no set that will improve the ratio, but until you determine that for a fact, you'll never know.
Yes it will take some detective work, it may take some measuring and disassembly, but so will any other solution.

If this helps.
I ran a Gear Vendors overdrive behind a NP205 transfer case for over 200,000 miles. The OD worked on the rear drive, ONLY. The GV circuit was interrupted when the transfer case was shifted out of 2WD.
I had changed the OEM IHC axles out for a set of DANA 60 with 3:73 gearing. The top speed against the governor was around 61mph. Engaging the GV gave a 1.28 advantage (1.28 X speed = new speed. )
GV does not work in reverse.. A GV is a little expensive. You are spending a ton of cash anyway and this works,
https://www.gearvendors.com/index.html

I did pick up a Spicer seven speed (ESO657A) overdrive trans with a .74 final ratio. With that, 43" 22.5 tires and 6.17 diff ratio I'll be 55mph at 2000 RPM, 62mph at 2200 RPM, 2400RPM=67 mph. These numbers were calculated using the Spicer gear ratio web tool. I'm satisfied with those numbers. I just wonder how well the old IH running gear will hold up at 60ish mph. I guess there has been millions of much harder miles put on the same gear in years past and maybe still today. I think I'll run this set up until I have to replace them; then I'll modernize it a bit. Now I need to locate a dog house to shelter that big DT466!

I'm going to replace the front bench seat with some nice newer bucket seats from a Ford super duty. I may also do the same to the rear for a walk through to the camper. I was thinking of building a nice big center console/doghouse. Anyone have a favorite sound damping liner that would also keep heat out?

I did pick up a Spicer seven speed (ESO657A) overdrive trans with a .74 final ratio. With that, 43" 22.5 tires and 6.17 diff ratio I'll be 55mph at 2000 RPM, 62mph at 2200 RPM, 2400RPM=67 mph. These numbers were calculated using the Spicer gear ratio web tool. I'm satisfied with those numbers. I just wonder how well the old IH running gear will hold up at 60ish mph. I guess there has been millions of much harder miles put on the same gear in years past and maybe still today. I think I'll run this set up until I have to replace them; then I'll modernize it a bit. Now I need to locate a dog house to shelter that big DT466!

Exactly how do you plan to get those 43" tires to fit the truck? A 10X20 wouldn't fit and it is only 41" What tire that will fit a 20: spoke has a 43" diameter?
Go back and re read the 1st 4 posts on this thread.

Well it does have the 10x20 on it now that are roughly 41"+. I have a set of 22.5 Dayton wheels that bolt right up. As far as clearance there is plenty for the 11R 22.5.

Still only a 41 in tire, 11x 22.5 and 10x 20 are exactly the same diameter for the same tread. Still nowhere near 43"
In posts on the 1st page you implied it had 9.00X20's on it. There has to be enough clearance for the tire to clear the springs and frame, and still have enough clearance between the tires on a set of duals.
Just because a 22.5 rim slides on the spokes doesn't mean the tire will fit are clear, or that there will be enough tire to tire clearance.

With that O/D and 11x 22.5 tires (if they will fit) would be turning 2287 RPM @ 60. It will give you a top speed of 63MPH with 2400RPM gov engine speed.

I have a IH bus that the 22.5 wheels are currently on along with 11R 22.5 tires. It has the same rear differential as the fire truck just different gears. The tires that I'm looking at have the following measurements. W=10 H=43 weight of 150lbs, according to the manufacturer. That's a difference of .5" each side of each tire. I realize that rim width affects the overall width of a tire and the sidewalls are wider than the tread. As it is now there is 2"+ between the inside of the rear tires and at least that before if have interference with the rear springs. Also plenty of clearance up front.

I can't seem to get pictures in the correct format or I would post some of the build and truck.