The Ersons when used on the Edel heads with ARP head bolts do need a slot milled into the rocker housing to clear the heads of the bolts. With the other stock style shaft mounted rockers it's best to use the ARP rocker stud kit with the Edels. The D2-AA heads were the only FE head to have induction hardened ex seats. But they are the same otherwise as the earlier C8-H's.