I've got the 2.78 rear in mine. I'm happy with the performance of it with just the base 350, but I'm more interested in cruising than performance.

HAHA, still don't pass many gas stations. If you are worried about gas mileage in these cars, you probably shouldn't have one.3

3.23 to 3.55 would give you a big boost in performance. Any higher than that and I think you would loose most of the cruising ability. Changing gears can wait until after you've got the engine built and back in, and you'll also really get to see the difference in performance between the gears.