The heads that come on the zz4 crate engine are milled for 58cc chambers and the cam has a very mild lift. Would you be able to use these on a 305? Where can you get them?

The heads are aluminum, and therefore even though they're the same chamber size and the same static compression, will reduce the engine's thermal efficiency. When using aluminum heads it's usually necessary to add about half a point of compression to make up for the amount of heat (energy) that gets lost through them into the coolant, that doesn't get the chance to get converted from heat energy into mechanical energy.

The cam gives valve lift of somewhere near .520". Some of us might consider that mild, some might not.

They will both work fine on a 305, as far as that goes.

You would get them from a ZZ4. Find somebody that bought one and is replacing them. They show up in the classifieds on this site from time to time. Alternatively, you can buy themdirectly from GM.

Would they be a good idea or is there something better?

The heads you've already got, if they're the factory ones (081 casting) are perfectly fine for the application at hand. I'd suggest porting them, maybe put larger valves in them, etc. etc.; it will cost many hundreds of $$$$ less, and run as good or better.

That cam is a decent cam, that you can pick up for cheap. If you run it make sure you upgrade your valve springs, and maybe your rockers too.

...and rocker studs, too.