The GT40X heads cost $629 a pair from summit racing. I would link you but thier catalog system prevents a direct link, check it out:
http://www.summitracing.com , enter this PN in the simmit part # search: FMS-M-6049-X303
Trickflows cost $1000, not GT-40's
I was mistaking thinking the P's were more expensive. But Summit sells them for $269 a pair.
My friend has the GT40X headed crate engine with the B-cam in his stang (345hp rating from Ford) and it pulls like it has 360 hp.
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-1993 Mustang, 900rwhp 03-04 cobra based 4v swap, full Griggs setup, project underway
-1994 Ranger w/ 427cid windsor (daily beater)
check it out!