keep your goals modest. ive never seen a 302 that actually got "good" gas mileage. obviously, fuel injection and/or an overdrive tranny will help a lot.
ive had a pair of 302 mavericks, and a 289 mustang. the very best i ever got was 18 mpg, absolutely "babying" the car. that was on the highway, doing 55 mph, coasting as much as possible, and no high speed passing.
that same car got about 14-15 mpg, driving it as i wanted to. it was a lot more fun to drive, and i got there faster.
i would expect everyone else's experiences to be relatively similar.
id say at worst.. (very poor state of tune) 10 MPG
very best.. 20 MPG