Welcome to CHR. The Turbo 400 is a non-OD transmission, so your final drive ratio is 1:1. You need to figure out what you want for your cruise rpm and go from there. I use the formula:
Cruise RPM = (mph x rear gear ratio x 336)/tire diameter in inches

Right now if your rear tires are say 28"OD you get (70mph x 3.08 x 336)/28 = 2587rpm. If you change to say 3.55's, then you'd have 2982rpm, getting up there. For me, I'd probably keep the 3.08's for cruising, or look for an OD tranny if you want to get taller gears for more pop in your acceleration. Taller tires will help drop your cruise rpm, but will hurt your acceleration.