Brad54 hit the nail right on the head. Sometimes it's more about the quality of life than MPG. My '66 C10 has a built 350, turbo 350 and '72 1/2 ton rearend with 373 gears. I'm at the point where I can't tolerate the low rearend anymore. By the time you get where you're going your exhausted and you know the high RPMs aren't doing your engine any good. I bought a good rearend out of a '72 Suburban with 308 gears for $50 and spent another $75 to get it ready to put in the '66. Dollar wise this makes more sense than an overdrive or changing gears. I've owned several '71 and '72 1/2 ton pickups over the years with 350/350 and 308s. From my experience that's a really good all around drivetrain combo.
