If you're not going to change the engine for a faster running one or the transmission to one with an OD then the only variables you have left are tire size, auxiliary transmission, and rear axle gear ratio.

Run the numbers here
http://www.onlineconversion.com/bigger_tires.htm

•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-