What I would do and I know how rare the 302's are since I owned a '68 Z/28. I would sent the 302 out for a rebuild and when completed, set it aside. In the meantime, I'd have several options:
1. GM crate 383
2. GM crate ZZ4 with fast burn heads
3. GM 2006 427 small block vette engine (lots of extra cash)
and have fun with the Z/28 and not worrying about destroying the original 302. The value of the car will still be there. If you ever decide to sell the Z/28, then you swap in the 302. Done.
