What ratio is your "open rear end that a friend had in his t-bucket"? Is your Overdrive shift cover a 4 bolt or a seven bolt? Either way the overdrive is .70 so in my case thats .70 X 3.90 rear end = 2.73 rear ~70 mph @ 2000 RPM. You want to have a good power band for the terrain in which you will do most of your driving, in my case the piedmont foothills. Here's some information you may find useful. Specifically, the ENGINE RPM CALCULATOR. You can't change the overdrive so what you can change in your case is the tire height and rear end.

PS, you're going to love that overdrive man!

~ Cosmo
1949 Chevy Half Ton
Rocinante, like Don Quixote, he is awkward, past his prime, and engaged in a task beyond his capacities.
"...my good horse Rocinante, mine eternal and inseparable companion in all my journeys and courses." ...Don Quixote, Cervantes
"If you come to a fork in the road, take it."...Yogi Berra
"Rudeness is the weak man's imitation of strength." ...Eric Hoffer