I recently bought a 66 Ford Mustang with 32k miles (perhaps 132k?)
The car is in a very good condition, no rust, drives and shifts well.
The owner, a nice elderly man, told me that the engine has been rebuilt 1000 miles ago by a mechanic. The latter one told him to simply use premium gasoline. He doesn't know though if the valve seats have been replaced. Should I pour lead additive anyway to prevent engine wear?
Thanks
That's a somewhat controversial topic. From what I've seen the general consensus is if the car is lightly used (not raced, no towing, limited miles per year) it's not necessary. However there are lead substitutes available if you are concerned about it. (You would not want to use actual tetraethyl lead even if you could get it because the stuff is extremely toxic and rapidly absorbed into the human body.)
For what it's worth I've owned quite a few 1960s and early 1970s cars since the removal of lead from gasoline decades ago, and still own a few. I've never used a lead substitute additive and have not had any problems. However today these are hobby cars that are not subject to the rigors of daily use.
Will adding lead additive damage the engine IF the valve seats have been replaced?
No.