In '63, I had a '58 Edsel, with a 345 hp, 410 MEL engine and a 2.91 rear gear. Cursing at 65 mph I could get 18 mpg. It had a Cruise a Matic and didn't lock up, of course but, we had good gas in those days. Also, it was a 4000+ lb, boxy shaped car.
I think a T-5 is to light duty, if you plan to romp on it very much. If you can, go with the 4R trans and a 3.89 or 3.74 rear.