Going over 55 mph is going to be more a function of gearing, than trying to overcome with brute horsepower. Your rear end is geared at 3.90 to 1, (assuming your '61 is a half ton)if you have a non overdrive tranny your final drive ratio will be 3.90 to 1 because the tranny in it's highest gear has a ratio of 1 to 1. An overdrive tranny with .70 over will give you a final drive more in the range of 2.70 approx. Or you could just change the rear end to a 2.73 if fuel economy is the issue. If you just want to be able to go 75 on the hiway, you need to find a 3.08 rear end. But then your off the line acceleration suffers. The best of all worlds (IMHO) is an overdrive tranny with the stock 3.90 rear end. You can get a good launch off the line in low gear,(even with the inline 6, for city driving) and as you get to the hiway speeds the O/D reduces your engine rpm so you can get up to 75 mph without redlining the engine. It's a matter of compromise. In my opinion, with a V 8 in your truck, while retaining the 3.90 rear end, you'll do 75 mph, but the motor will still be screaming itself to death, and you'll have trouble passing a gas station. Just my .02
