I had a 66 Charger 383 auto. 8 3/4 rear posi with 3.23 gears and 295/50/15's. It was ok for 55 to 65 mph hyway driving, but for the freeway no way!
My 68 Chevy CST pickup has a 383 stroker (350)and 400 auto trans. No need for high dollar O.D. stuff. 3.08 gears with a 32" tire and 80 mph is as easy as 2600 rpm. At 4600 RPM, well, you do the math, let's just say were flying!!!