Gas Mileage on a Chevy 350

what kind of gas mileage can you expect from a 1965 chevy 350

I do not think the small block Chevy existed in 1965. I believe '68 was the first year.
It is very possible a later engine was swapped into the '65 in question, although this would affect it's value.

Mileage can vary enormously. What kind of '65 is it? Chevelle (mid size) or full size? Wagon, coupe or sedan? Corvette?

Is the engine stock? 2 barrel carb or 4 barrel? Auto or standard?

All these factors affect mileage and without more info, no one can give you a remotely meaningful answer.

Correct, the 327 was the largest small block in 65.

I have a beater 69 C10 truck with a tired 350. I put 3:08 gears and 30 inch tires out back and she gets 17 mpg freeway at 70 mph.

The first generation of Chevy Small Block came into existance in 1955, of course the small block 350 came many years later.