If the steel EQ heads aren't available, guys have had pretty good luck with the Speedmaster aluminum heads. Your goal would be easily accomplished with the non-CNC version, either bare or with valves and springs. Pay a shop to check them over and tune up the valve job and they'll flow enough to make 500hp on a proper combo. 400hp should be like falling off a log for a 408. A set of bowl ported J-heads will do that with ease.