I have used both OEM and repro HP manifolds, and found the repros to be superior. Two reasons- Originals are most commonly the C7ZZ type, used by the factory in the latter part of 67, and for nearly 20 years over-the-counter. They have a deep notch on the upper left corner of the ports to clear the smog injectors, and are more apt to leak. Functionally, the core support "bungs" on the outside of the OE are also on the inside, right in the radius of four of the ports, blocking optimum flow. Repros use newer techniques, so the bungs are on the outside only, purely cosmetic. Either way, if you port-match your heads to the large HP manifolds, and clean up the roughness or bungs inside the manifolds, they work pretty well.