Usually the square or rectangular hole is for a locating tab, with the round hole being for the fastener. Pretty sure the fastener would be 5/16-18 thread, likely self-tapping if there's no nut welded inside the rail, but if you see what looks like a square plate behind/inside of that hole, that's for a self-tapping bolt.
I have a pair of tailpipe hangers for a'67 GTX; don't know if they're the same for Hemis & 440s.