As Baddad has said, the manifolds should be fitted with studs, not bolts. The thread is 7/16-14, i.e. standard coarse thread.

The fact that you found bolts used simply indicates that someone improperly reassembled the parts at some point.

The threads on the manifold are regular threads, but the studs have a special interference thread design to keep them from unscrewing. Double nut them to insert (poor quality replacement parts may not have this feature).

You can probably still get the studs from your local Ford dealer.

Use never-seize on the nuts.