Install a Tremec T56 Magnum in your A-body

And a real good part of the T56 is that your rear end will never need any lower ratio than a 4:10. My truck uses a 3:42 and with its tire size comes to 3:53's which @ 2000 rpm in six is 93 mph, so driving @ 2000rpm in fifth is around 70 mph. It's gear ratios on the transmission are 2.66, 1.78, 1.30, 1.00, .74, .50 and Torque Rating : 450 lb-ft

Hope this helps those who need to know :)