318 heads, 302 casting. Mystery holes?? (Solved!)

With a set of headers they'll also leak badly, and make a really bad exhaust leak sound.

That's just flat out untrue. With GOOD, STRAIGHT header flanges, GOOD header gaskets like Remflex and a GOOD set of header bolts, they will not leak.That may have been your experience with inferior parts, but not everyone has the same experience by FAR.