The Allison is supposed to be strong, but it is also heavy, and the one you have is not so modern any more. It is also not overdrive.

Do you need overdrive to go 80 mph?
A Dana 70HD is available with 3.54 gears as your fastest option.
Lets say you run 8R19.5 tires, as they are good truck tires, they look somewhat better than 16" ones on a 2 ton truck, and are 33" tall.
Then use this site to calculate rpm at 80 mph http://www.onlineconversion.com/bigger_tires.htm
looks like without OD you will be turning 2,900 with this combination, is this acceptable for either engine?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-