Hello, assuming you have 27 inch tires, a 3.90 diff, at 65 mph you are cranking at 3100 rpm. For sustained use, that is probably to much. If you had even a 3.54 diff, your rpm would still be 2863 at 65 mph.
With the R10 OD trans, you could no doubt travel at those speeds ie 65 to 70 mph at 2200 to 22500 rpm, which is a lot more relaxed for this engine.
Your present set-up was designed to travel comfortably at 55 to 60 mph.
I know someone who back in the 60s had an early 50s Plymouth with a 4.10 rearend, he was heading to Grand Beach, 70 mile north of the City, he was doing 80 mph most of the way, he blew the engine unfortunately that day and had to be towed home.
You could always convert to a T5 trans, from an 80s S 10 truck, with some mods to your bell housing this trans will work, in 5 th gear you should be able to cruise nicely at 70 mph.
My car, a 1947 Chrysler Coupe has a 4.30 diff, which was transplanted way back when, I have a 3.54 waiting to go in it's place. With this present diff, I would not venture over 50 mph, so no highway cruisng right now, even at 45 mph she is getting up there.........Fred
