When you get ready to do this you might as well plan on changing the rear axle as well. Otherwise you'll just be wasting the power that cummins will make. These trucks were geared extremely low to make use of the low power outputs of the 235. I have a '50 4100 (1.5 ton) and it has a 6.17 gear. The 2 ton trucks could have an even lower gear. About 40mph will be the top speed - especially with a governed diesel.
