From how thus thread reads, Originally the plan was factory heads only.
Then a consideration was made that went something along the lines of "ported factory iron has sweet ****-all resale value, this is particularly a problem for shops. How about leting us use aftermarket heads so we can build something we could actually sell afterwards?"
So a change was made and edelbrocks and speedmasters are now in.
But still not permitting EQ/RHS heads despite having comparable value and performance potential to both the edelbrock and speedmaster
Presumably because the EQ/RHS are more work and less economical to port, and not available to purchase new.
....which also applies perfectly to factory iron heads, which are allowed.