My first car was a 64 Impala with a 283 and a 2-speed powerglide, it had 3.08 gears in the rear and got right around 21-24 miles to the gallon. A 283 with a T-5 and 3.55 gears would probably do real well as far as mileage and power. Good luck with it, sure wish I'd had access to a site like this when I was 18!
