Consider the speed you want to drive at, the RPMs that the engine will enjoy running at, tire size, transmission high gear ratio, and your axle gear ratio.

I believe that proper gearing should come first, sure beats having a truck you can't drive how you want, or can't afford to feed.

I'm almost certain your Allison is not an overdrive, this could be a big problem if you are stuck with the axle gears you have now, as they are probably really slow.

This site is great for figuring speeds
http://www.drivetraindirect.com/t_gear_calculator.htm
and this one I just found, and I may like it better...
http://www.onlineconversion.com/bigger_tires.htm

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-