I did some experiments years ago with a stock 250 in a 70 Nova.
I put a 2v Rochester from a 307 on the stock intake with an adapter. Got about 3mpg over the stock 1v and a lot of throttle response and acceleration, though no times or anything. 1v was in excellent condition and I tried one from a 292 also. The 292 made more power, no better on economy. The 307 2v was hands down the winner. I had stock exhaust with a "turbo" muffler.
At the time, I had NO money so tests were with what I could get CHEAP!!!
Also, the vacuum advance made 3-4mpg difference. Timing was tested at every step but I don't remember the figures.
I think I got up to 22mpg with a Saginaw Vega 4 speed and stock rear gear.
