Certainly, you've ruled out flatness possibilities with the heads and covers by now. How about the PCV system / blow-by pressure?
Way back if we didn't want corks leaking, we cleaned the surfaces with trichlor and glued 'em up with that black paste gasket sealer then let it setup overnight.
OOTB,
Assuming "thin", as in height. Are the Felpros thin enough to consider doubling up?
