61 1/2 ton should have either 4.10 or 3.90 gears, iirc. Another factor is tire size, specifically the rolling radius, measured from pavement to the center of the wheel. You double that distance to get the tire diameter for the formulas. Cruising rpm for the stock 235 is 2,200 to 2,700 rpm. Above that for extended times and your rods start to stretch and fatigue. Here's one link to a calculator, there are a lot more out there.
http://www.ring-pinion.com/calculators/calculatorsind.html

Fred
52 3600
69 C-10