Gosh...getting a cast made for this DOHC head would cost an INCREDIBLE amount of $$$...but really that would be the only way to ensure that there was no integrity and strength sacrificed. Eventually you would more than make back your money as potential buyers would feel very comfortable knowing that the heads are cast and not two welded KA heads with questionable internal welding.
A very well ported and polished P90 flows some really impressive numbers..I remember seeing a L28 with a P90 pick-up almost 30wheel hp just by switching heads and same cam..very impressive and this was on a NA motor.
Naturally your head will still outflow a really well manicured P90...but there is alot of testing that you would need to do to make a potential buyer feel confident about spending the $$ on your DOHC head and the unique intake as well. On ZCAR you said that the ball park would be around the $5G mark...that is more expensive than buying a RB26 motor, tranny, harness and ECU. Actually you could probably get a RB25 Skyline front cut for close to that - not trying to sound negative or disuade you and your excellent work.
A fully rebuilt and extrmely very well ported and polished P90 will run about $1100.
Hope to hear more from you and updates on your great work.
Regards - Yasin