This link can help you decide on heads-
http://www.wallaceracing.com/headsearch.htm
But even then, after 40+ years, a lot of heads have been altered, so you need to verify the chamber size with any head you find. There are a lot of choices in the 72-85 cc range that would work on a 350. Either pick the heads based on a cam you like, or pick the cam based on your available heads.
You can get a cam that builds pressure for a low compression head, or a cam that reduces cylinder pressure for a high compression head. If you are buying new pistons, you can get dished pistons and use the heads you have, or maybe shape the chamber in the head to get a few more cc's out of them.
My suggestion would be to look for heads around 72 cc, with screw in rocker studs. Big valves don't matter as much on a 350. But any head can be converted to screw in, so don't automatically discount pressed in studs if you get a good deal.