Jack do you know if the valve locations are offset??? It is common in sbc performance heads to put larger valves in they move the exhaust valve over so that they can also move intake valve away from edge of chamber---------Glad your sure that you don't have a geometry problem----

Whose/what company rockers are on engine and what ratio????
I've seen a few cases of the fast burn heads having valve seat ring get loose because of temp differance in aluminum head and as it moved down it holds valve open some creating an odd clattering noise before it eventauly creating a mess in the cylinder with a dropped valve, pieces then getting into intake and over to other cylinders-------