which heads for a 408ci?

No, the Eddy's don't offer more potential than the Indy's for sure, however there are other heads out there, W5's, W7's, W8's, and W9's. Not to mention the B1 and Brodix MC heads along with the large port Commandos (which are my personal choice for new castings for use with W2/W5 valve gear.

The OP's original requirements do not need any of this more exotic stuff though. He can achieve 520 HP from a 400 cube small block with heads that flow far less than these. My suggestion is to do it in aluminum as it takes weight off the front end and you can run more compression on pump gas.

MP aluminum Magnum R/T's will do the trick with mild porting and you can run a smaller cam and use cheaper Chevy 1.6 roller rockers. It's a win/win situation. Just be sure you are going to be satisfied at that power level to make it worthwhile. Otherwise buy heads with better potential to start with.