which heads for a 408ci?

No doubt Indy's work, however I think there is more potential in other heads properly ported. The Indy head is a good head out of the box though there are many ways to skin a cat. Our OP was only looking for 500 to 525 HP. This is achievable with much lesser heads.

On a side note why would you give up so much potential to run heads out of the box? You could have saved the dyno time money and spent it on head porting and been far ahead of the game.

Don't get me wrong, dyno's are great tools for those who need them. Engine builders, Pro Stock, Comp Elim, Super Stock, and Stock racers I can see. But the average drag racer really has no use for a dyno as they are not looking for that last .001 of a second. If you can use drag strip ET's or MPH in paper calculations than you can evaluate the validity of changes made.

Just my $.02