With a stock rearend, trans, your mileage would be pretty miserable, i would guess 12-15 mpg. Myself, budget minded, I'd go 70s Nova auto rearend, turbo 350 trans, tallest wheels and tires I could find for a decent price. Unsprung weight- tires, wheels, tire width would all be a factor. A Nova or Camaro subframe or Mustang 2 front suspension will lighten up the front and give more stopping power. Your truck is about as aerodynmic as a brick, you can't expect miracles but would definetely be a cool ride. Maybe around 18 mpg with something like this. good luck.
"When I was younger, I could remember anything, whether it had happened or not." - Mark Twain