Old fire truck

Ok, new idea!
Considering the intended use of this vehicle and the concerns raised about reliability, driveability, economy and safe modern travel speeds.
I can get a 6.0 GM gas engine and six speed auto package for about 6k ready to install wiring and computers ready to go. I'd just need to fab engine and trans mounts and hook up fuel and power.
This is a major departure from what I wanted to do. However I need this thing to be solid. A few questions for you.
1. Will the current t-case Rockwell T223 handle the increased rpm from the trans overdrive? final ratio is .667 current final ratio is about twice that.
2. Will the differentials tolerate the increased speed and rpm? I know the front is a Napco and the rear is IH with very low gearing 6.17 maybe lower.
I think with that combo speeds of 70+ would be possible but I'd limit speeds to 60-65. That's a big old rig to have going 70+

I think the transfer case will be ok, heat would be what killed it, so may be a temp gauge in the case would be in order.
What really concerns me would be the drive shaft speeds. There is a critical speed for a drive shaft based on size, weight, how long and most important U joint angle. Run some calculations on driveshaft angle and rotation speed, and take one into a driveline shop and ask if it can turn that fast. Longer and steeper angle and the slow they have to turn. Same for heavier and larger.
All that gearing up and back down again is going to put a lot of energy into heat. Both drive axles, the transfer case and the trans.
For $6K + you could re-ratio both cog's if faster ones are available and still come out money ahead.
I don't know of any Medium 4X4 trucks that are set up for running highway speeds, 55 tops for the ones I see. Most are used for powerline work and speed is not an issue.

Good stuff, I didn't even think about the drivelines. I think you're definitely right about the re-gearing as far as cost. I'll do some more research into the differentials. I know that the rear is RA351 and I believe the front is a Napco. I'd be perfectly satisfied with 55, the 70 figure was a rough calculation for the conversion I would never drive it that fast.
I have seen gear sets for the rear but the Napco is a different story. Thanks for the help!

Check with Jeff Bradshaw from Redneck Restorations out in Oregon- he's on youtube a lot under ElderlyIron. He seems to know all about the Napcos. I'm sure there are others as well.

Good stuff, I didn't even think about the drivelines. I think you're definitely right about the re-gearing as far as cost. I'll do some more research into the differentials. I know that the rear is RA351 and I believe the front is a Napco. I'd be perfectly satisfied with 55, the 70 figure was a rough calculation for the conversion I would never drive it that fast.
I have seen gear sets for the rear but the Napco is a different story. Thanks for the help!

I don't believe NAPCO made any of the ring and pinions used. I know Coleman didn't. They modified others carriers for front applications. One reason is they had to match the ratio, front to rear.
1st step would be to look on the ring and pinion for makers numbers. Although IHC DESIGNED their own rears, they didn't PRODUCE their own rears, they were made by Spicer for IHC. I'd be looking for a diamond Spice trade mark along with an IHC part number stamped somewhere on the set.
I think Marmon and Fabco did produce their own carriers, not sure if they used someone else's gear sets.

Ok I've setteled on the engine and transmission. I've picked up a GM 6.0 gas and 4l80e transmission. I'll keep the Rockwell T223 transfer case and leave the gearing in the differentials as is. Highway speeds and fuel mileage will be more than acceptable with this set up. It isn't true to IH but given the intended use for the vehicle I think it's the best compromise and will be reliable.

Don't forget to calculate the "critical drive shaft speed" and be sure your not about to exceed it. Nothing like a driveshaft coming apart at speed to ruin your whole day!

Do you have a plan to stop that great pile of parts? At 60mph there is a lot of kenetic energy that has the potential to melt the brakes.

Brakes on trucks in that GVW range are spec'd for the GVW and not the speed the truck is geared to. If the brakes are well maintained they should be good for 60-70, not to say you can't push them beyond fade, descending a hill. Brakes on road tractors are not bigger if the truck is geared for triple digits or 55.
You used to be able to spec "mountain brakes" which were an inch wider than normal brakes, but that has been long gone since engine brakes became more or less std on trucks.
A little common sense goes a long way. 16.5x 7 brakes are fine for 80K on the level at 70, not nearly enough at that speed descending a 7% grade.

I've given thought to the brakes and have decided that if they were designed to stop the truck and load. The miniscule weight of the camper shouldn't be a problem with another 10-15 MPH. As far as critical drive shaft speed, I'm not sure I know how to calculate that. I understand the idea but not sure where to start. The truck is relatively short as far as loadstars go. The rear shaft has no carrier bearing and would be only slightly longer than an average modern longbox crewcab. I think with the auto trans that will get even shorter. I imagine length of shaft has a bearing on speed it can turn? I will have drivelones built and balanced at a shop that specializes in that area.