Borgwarner 3sp overdrive transmission was offered up through 1972 so the solution is both cheap and easy. This gives a deep gear for starting off or load hauling but a shallow gear for highway cruising. Would be better if a 4.56 as the final would be 3.20 in OD which is a good cruising gear while the 4.11 is a bit shallow at 2.88 in OD. A 57 3100 we built with a 235 and OD was fine on level highways which covers most of Texas but not a good hill climber as the factory gear in a 57 is 3.90 which gives a 2.73 final in OD. Factory bolt on parts can fix your problem.

Evan