which heads for a 408ci?

They are pretty much a W5 head without the raised and extended intake port. So you don't need the china wall spacers or special intake. The actual potential flow didn't suffer all that much either and may have even gained some lower lift flow. The W5's require some work to match the rest of the combo to the heads, or it may even be more accurate to describe it as crutching some portions of the combo because the port volume is so large low speed flow is seriously comprimised.