What Chevy engine heads can you modd your 305 with?

Because of the 305's small bore, you're pretty limited. Large valve heads (2.02/1.60) won't work on a 305 because the valves will hit the block itself. If it's an older 305, your best bet would be the old power pack 283 heads. They flow better than the older 305 heads. Now, if it's a vortec engine, you probably won't do much better than what it had stock. If you are really trying to go fast, the best thing you can do is junk the 305 and replace it with a 350 or a 327.