I recently purchased a 1981 C10 with a 350 crate motor and the original transmission. I'm getting around 14mpg and figured if I had an overdrive or a better overdrive that the mileage would go up. Does anyone have any suggestions on which overdrive transmission I should look for? Also, are there gear changes in the rear end that I could make that would help? I'm not looking to squeal the tires since this is a daily driver, so I don't need something too heavy duty.