http://www.onlineconversion.com/bigger_tires.htm
9R22.5 tires which should fit nicely in the 51 fenders are 38" tall
The original 4 speed is a direct drive; 1:1 ratio in high gear.
A later 235 is comfortable on the highway at a constant 2,400-2,500 RPM in my experience.

Plug all that in and it spits out 53 MPH

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-