Have you considered an overdrive transmission like a T-5?
What are your plans for the truck?
Will a 3.42 rear give you the speed and rpm you want?
What will you do for a parking brake? Many SM420's have them on the back, that may be the way to go.

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-