What size 45 truck? I guess a 1.5 ton right?

No, no kit that I know of.

If you are up for an entire axle swap then you will have some options. Check out my pictures on the link below, covers a few different axle swaps.

What engine do you have? If it is only a 216 then faster gears will only hurt you while hauling a tractor, unless they are pedal tractors smile To go fast and haul a load you need power..

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-