Some folks are of the opinion that 305 heads with their smaller combustion chamber will raise the compression ratio (true) and increase performance (generally false.)
There's another thread in this forum where it's been discussed at great length. But the bottom line is that 305 heads are designed for a motor with a much smaller bore than the 350 and as a result have smaller valves and create a weird combustion chamber shape.
I agree with 4.3 about needing more info before making suggestions about fuel mileage...but just as a general suggestion, you might consider some 350 heads...882s or 883s are common and not all that bad, for smogger heads. With 1.94 or 2.02 intakes they flow decently. And if you go with a 4-barrel carb that has small primaries and large (relatively) secondaries, you can get pretty decent mileage if you keep your foot outta the carb and don't open up the secondaries.
I run a Carter 625 AFB (basically the same thing as an Edelbrock 1406 600 CFM electric choke)on my 292 and very rarely open it up. I get about 15 mpg with 4.10 gears. I'm ok with that.
Probably the best thing anyone can do to improve mileage is a higher (lower numerical) rear gear. But performance can suffer. I have an 80 El Camino with a mild 400 smallblock that had 2.73 gears in it and it got 15-18 mpg. But it was a dog off the line. When that rearend grenaded I put a 3.73 posi into it. It's a whole lot better off the line (burnouts are a lotta fun) but the mileage dropped to single digits...and since it has a 1:1 top gear (TH350 trans) it winds too high to be used comfortably (for me, anyway) on the freeway. So I have an overdrive (TH200-4R) that I'll be putting in it. That otta bring it back to the 15-18 range.
Good luck with your Burb.
