Absolute best for me was 18MPG. Conditions were sea level, flat road, 50MPH, cool evening, no stop and go at all. Full tank of gas lasted a LONG time. /forums/images/graemlins/smile.gif Combo was "stock" 350/Q-jet/465/205/3.42/33's.
I broke a rocker arm stud in the new motor before I could try a similar extreme mileage test, but I averaged 14.4MPG with my Vortec headed roller 355/TPI/465/205/3.42/33's. Conditions for that test were a freeway drive of about 1.5 hours at 60-70MPH (speed limit change) up to the pass and back, from sea level to 3200 feet in the process. After the 45 minutes up to the pass, spent about 1.5 hours climbing and descending logging roads, playing around at about 4400 feet. Drove back home and filled up to get the 14.4 number.
I suspect with similar conditions to the first test the new setup would easily best the carbed setup, but those conditions aren't easily duplicated.