An STC (or other means of approval) is certainly required for any modification that major. One source is LyCon in Visalia, CA. They did a friend's 7GCBC and performed their port/polish and flow match to the cylinders at the same time. His 150 dynoed at just over 180hp when they were done. Having flown the airplane before and after, I can say it was well worth the money. Performance is outstanding.
Hag
