Old fire truck

Things the driveshaft shop will need to know, length, U joint operating angle , shaft speed. With those three things, with the shaft size and weight they can tell you if you are in dangerous territory.
even if you are below critical speed, the faster it turns the shorter life of the joints.

Excellent advice thank you!

https://spicerparts.com/calculators/dri ... calculator

That's perfect thank you!

Ok guys, it's been a while since I last posted. I have been making progress on the truck and need a bit of help. I've gathered up some parts for the drivetrain. I have a mechanical DT466 that is currently mated to an Allison MT643. If your recall my axle ratios are 6.17. I just purchased a Spicer ESO657A which is a 6+OVERDRIVE transmission!! That should solve many of my problmes. I also have another test differential from the donor bus. It has 4.38 gears and hydraulic disc brakes. If love to find out have gears made that match that for the front differential which is a Dana/NAPCO S9000. Any ideas for that front differential? Also what might I expect to get for a good used Allison MT643?

That Allison MT643 is nearly obsolete. They are only 4 speeds and the top gear is likely 1-1. Many persons prefer an overdrive top gear.
I used to rebuild them at work and I had a small re&re business in my shop. I used to pay $300 t0 $400 for a core. If the transmission is not known to the buyer/rebuilder, it is a core. If there is not a local buyer, the shipping may make your Allison worth scrap in weight.
That Allison was a tough transmission. With proper cooling it should last several hundred thousand miles.
The Allison is very much like a GM TH400 or more like the TH475. I have a TH475 behind a Cummins 4bta. I built an adapter to fit a 241c to my TH475.
Parts are easy enough to find.
If this were my truck, I would be keeping the Allison. Nothing works better in loose sand and mud than an Allison. An Allison is easy on the driveline.
I do not need to be reminded that this is not my truck, etc, etc, AdNauseam

https://youtu.be/1UuHEbHGssI

That's a great demo video! I'd love to run an overdrive auto. Unfortunately without replacing both of my differentials I'm stuck with 6.17 gears. As I can't seem to find a replacement set of gears for that Napco S9000 front diff. I'll need overdrive and don't want to add a computerized transmission or computerized anything to my build. So I'm gonna run that Spicer with the differentials that I have.

If I could regear my current differentials I'd just run the MT643.

I emailed Jeff at RedNeck Restorations (thestaff@elderlyiron.com) who specializes in GMC/NAPCO trucks. He is not aware of any supplier of different gear ratio sets for the NAPCO front diff. He did say "I do have a large, OD auxilliary transmission for $200 plus shipping! It's OD, direct and 2 under drives." He's in southern Oregon.

I like the sound of the Dana that overdriveSteve referred to earlier.