I ran a small Plym flat head motor with an Isky cam, Offy intake, split exhaust and edmunds head in a '48 coupe with 3.9 rear gears and 29" tall tires and cruzed at 75+mph with no problems and got 20 MPG at 65 mph. Couldn't start in 2nd gear, but pulled strong from 50 mph to 75 mph. Made a believer about 6's of a friend with a 289 mustang.
For the record, my stock drive train '49 Wayfarer Coupe has no trouble reaching 85 mph. I don't know what RPM's it's turning at that speed. I'm pretty sure it will go faster than that if I want to push it even harder.
It's kind of fun to surprise guys in new full-size pickup trucks, jeeps, SUV's etc. by walking away from them up some of the long hills we have in Western Oregon.
I think there are still pics of it in "rides".