That is one fine Olds, always loved the look of the 50s Olds but unfortunately never had one. See if you can find an Olds forum on line and they will have lots of info on your valve train noise. I remember that the Olds all tended to have that issue and the fix is most likely not that costly to repair. My guess is there is no reason for an engine rebuild Do a compression check and that will give you a good indication of the true condition of the engine. Good luck with your new car and enjoy the Ride Sent from my iPad using H.A.M.B.