I don't know about the 307, but the difference in mileage between a 305 and a 350 would be little at best. Besides, driving habits would have to be considered.
Now, the 305 ain't as cool sounding rolling out of your mouth and they have been known for small exhaust and intake ports, but consider this.... just how fast are you trying to go? Are you goin to be racing it or displaying speed a lot? Then, I'd say, 350. Lot's of god, cheap tricks for bumping one up. If you want a nice, still kinda quick, easy to maintain AND replace (if you had too) engine, then the 305 could be what you want. I was going to go with a 350 out of a Vette, that a salvage guy was holding for me. He wanted $650 for it. He called and told me he had a couple 305's I might want to look at and they were only $400 each. Mine runs great. Moves my car plenty quick and is STOCK. Heck, I'm even running the stock Quadrajet carb! Why? Because it works. I haven't even rebuilt it yet!
So, you got some fuel for thought. Good luck. Oh, another thing.... if you get the 305, just SAY it's a 350. Chances are, no one will know the difference, anyway!
What are they gooing to do, write down the casting numbers and check them out!? Please.:lol: There's a whole lot of guys TELLING how much fancy speed stuff they have in their engine and are running stock or near stock gear!
Can anyone tell if an engine has a certain brand/type pistons from the outside? I kinda doubt it.