I recommend using the '68-74 318 heads which have less crack problems and have open chambers. You can use the later, more plentiful intakes with them. This is if you plan on boring the block and using new pistons with a higher compression ratio to make up for the open chamber heads. If you are keeping the stock pistons, I'd get some later 302 heads and modify them to fit the stock exhaust manifolds. (not a major job). As mentioned, the other option is to get a 318 to bolt in and save the stock engine for a future restoration. There are some minor mods to do that but nothing serious.