Last year I did a similar engine, our RHS heads with pushrod pinch porting only, 10.2:1 compression, our 251 solid FT cam on a 108lca, Scat kit like yours, airgap, 750DP, TTI step headers and 1.6 PRW rockers....it made 521HP at 6200 or 6400rpm, can't remember and 527TQ around 4700rpm.
So it would seem 500 should be attainable. 34° total timing seems to work best most times.
Brian