For an auto, I'd probably run 3.21's in a 8 1/4 or 3.23's in a 7 1/4 rear.

Look to getting the lightest parts possible.

I ran a 360, fresh but mild build, with 2.94 gears and the OD 4speed manual...did GREAT with mileage (best trip, 55mph was 26mpg...tended to average high teens and low 20's). The large torquey engine did fine with the super tall gearing, wide ratio's. This was in an A-bod, with an 8 1/4 rear. Stock compression, MP .450 cam, performer intake, holley 600, hedman headers, MP ignition in a 76 valiant 4 door.