I think the factory casting - but like Kid mentioned - the intake and exhaust need to have the capacity to be enlarged vertically but retain the factory manifold mounting points. Basically - it has to use available manifolding in my opinion to have any market viability but it aslo has to be able to be tweaked to feed the larger displacements as needed. I'd start by looking at the largest intake valve you can fit without hitting the bore of the block, fill the rest of it up with exh valve size. Give it a heart shaped closed chamber design as small as you can, and enough meat around the deck to stay rigid under boost. Keep the valve angles, keep the rocker design.
The hardest part will be keeping the cooling jackets decent enough to provide even cooling accross the chambers. Not sure on a billet head how they'ye machine inside there...
