For reference, I just lowered my gear ratio to 4.30's with 28" tall rear tires. It spins 2500 rpm at 75 mph with the torque converter locked up in 4th (.67 OD). There are plenty of online calculators to plug your numbers into and get a reasonably accurate result.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)