I once had a '66 that was a factory 327 and Powerglide. It was a nice driving truck, even at speed on the freeway.
I swapped in a mild 350 and when driven the speed limit it got 16 or 17 mpg on the road. And it would comfortably exceed the speed limit.
Probably the most helpful part of this story would be what the rear gears were, but if I ever did know 20-some years later I sure don't remember.
