If they are indeed 1967 920 heads they will use a 3/8" bolt. I believe the 64-65 heads for the 273 used a 5/16" bolt at a different angle. As long as you are not mixing the early heads with a later manifold or vice-versa, you should be OK. Any 2-BBL 318 manifold should fit, if you want to move up to a 4-BBL there are several aftermarket options that offer the smaller 273-318 port size with the 3/8" bolt.
