When I switched from 4 spd. to 5, the overall mileage didn't change enough to be noticeable.
I can get close to 20mpg on the highway. It is a 350 re-built to original specs (2bbl) with a quadrajet, 3.08 rear and .63 overdrive. Around town it goes way down.
I didn't do the transmission swap to save money, I do a lot of highway travel and don't like cruising at 3500 rpm.