There are four types of heads to choose from (I think). They come from a bus, 411/412, or 914. I'll start with the best and work back from there.
The 914 heads can be told apart because of the 3 studs instead of the four like the others.
If you can, get either the ones from the 914 or the 1.8L carbed engine (I think) these are the best for doing a carburated engine because of the valve size and that sort of thing. I'm not much for explaining things, so you can look the rest up on another site that knows more about this sort of stuff than I do.