PS... 500 HP gross/400 at the wheels is doable with a 440 with pump gas. Piston, ported aluminum cylinder head, quench, cam and intake all have to be matched up.

There is probably more in your combo with tuning but unless you know more about what's inside its hard to tell how much. If it were mine, I'd enjoy it for a while and in the meantime take some time to plan a higher power build from the ground up. I bet it moves that car well as it is now.