If you have access to an auto parts recycler, they’ll have a computer database of what parts are compatible with what cars and engines. That’s probably where I’d start researching this problem. In my experience those folks are very happy to help. I guess they’re thinking with the info they provide you’ll soon be back and buying some parts from them. I’d also buy some of those classic Mustang restoration magazines I see in the bookstores, magazine stands, and look through the advertisers. Some of them will probably be selling heads, and they’ll have a lot of experience with this sort of thing, as they deal with it every day.