Only certain heads are legal for each combination. Factory heads, obviously, and whatever replacements have been approved. Some Edelbrock aluminum heads are legal, with a weight penalty, but they require a lot of work ($$$$) to be legal - smaller valves, guides, etc...

In Super Stock, "porting, polishing, welding, epoxying, and acid-porting permitted. Grinding and polishing in combustion chamber permitted."

"Cylinder head must be able to hold combustion chamber, intake and exhaust runner volumes per NHRA Specifications. Any aftermarket steel valve permitted; must maintain stock head and stem size; titanium valves prohibited." "Valve diameter permitted to be +.005-inch or -.015-inch from published NHRA Technical Bulletins."

Basically, stock size valves and OE/NHRA spec port and chamber sizes, plug and valve locations. You can do what you want with that volume. Makes for a lot of work and some interesting port shapes.

Also makes it easy to get 5 figures wrapped up in good set of heads from a top shelf guy. And then there's the valve train...and the intake...