Well, someone not long ago posted his mpg with a flex fuel Explorer -- 18 - 20 with gasoline, 11 - 12 with E85 if I remember right. I assume said Explorer was made for 87 octane, so people with higher compression engine (for 92 octane) would get better results. How well a '73 bigblock run on an ethanol rich mix, I do not know, but my gut feeling is that the cheaper price will not make up for the mpg loss.

In any case, "conversion" with a carbed car should be a lot easier than with a computer controlled EFI system, so just follow Dino's advice, and try it. And don't forget to post your results here.