I'll share my opinion (you know what those are like). IF the car is otherwise entirely original AND your main goal was to preserve max value to that fairly rare buyer who will pay the highest prices seen for originality, then Yes, the swap decreased its value (and probably nothing short of putting the numbers matching 351C back into it can reverse that). For all other buyers and purposes, No. I would also say that the southeastern US is also a more challenged location to sell a Pantera for top dollar anyway, though. To really attract that top dollar on any style Pantera anytime soon, you may have to consider then consigning it in CA or auctioning it if it could be returned to being very original.