I have a 59 gmc 1/2 ton with a 447 inch pontiac motor (1959 pontiac 389 bored .030 and stroked 1/2 inch) This motor destroyed the factory rear I had with 3.38 gears. I replaced the rear with an aftermarket 9 inch with 3.70 gears. I swapped the 4 speed granny low with an NV4500 5 speed. I recently changed tire size from 28 inches to 31 inches, and plan on changing the gears to 4.10 to make up for the taller tires.
OK, I gave to much background. Bottom line my stock frame is holding up fine to the torque (about 450 lbs or more) You will likely be fine too.
3.73 gears might be too low for comfortoble cruising with a 4 speed. You might consider higher gears in the 3.08-3.38 range. Depending on tire size, 3.73 gears may be too high for an OD. The factory 3.90 ratio works really well with an OD transmission.
Mike B... The 20 mpg barrier is tough to break in our trucks. They simply push too much wind. Good luck to you in your attempt, but I suspect 17-19 mpg on the highway is closer to reality. I get about 14 mpg at 70 mph in my 1/2 ton long bed. 70 mph at 2050 rpm currently, will be 70 mph at 2270 when I change to 4.11 gears.
