Here is several head designs GM sells

12480090
Bare C5R Racing Cylinder Head
355-T7 'as-cast' Aluminum racing head
Professional porting and machining of combustion chambers required
No seats or guides
C5R rectangle port design requires aftermarket rectangle port intake manifolds
Designed for 2.180/1.625 valves
11' valve angle
Machined for 1.625 diameter valve springs & .500 guides
Designed for big bore (4.100 min) LS7/C5R/LSX blocks
210cc 'as-cast' intake ports
70cc 'as-cast' exhaust ports, same as production LS6
30cc 'as-cast' combustion chambers
All fasteners are metric
Valve cover rails have O-ring groove for .125 O-ring
Capable of over 800 horsepower!