mpg on my 340

No matter how much or how little alky is in your fuel it WILL DROP the mileage, because you must have more alky to make the same power. Ever seen an alky carb? They have HUGE jets compared to gasoline.

Retarding advance will drop mileage.

A change of camshafts is likely to, IE I bet it's hard to beat the original stockers for mileage.