[CSX 4027]

Your car is running with the timing retarded.
It is in no way too lean. That has little or nothing to do with it.
By running retarded you are actually running too rich. You are igniting the mixture outside the pipes therefore, the pipes are getting excessively hot and possibly are glowing. It is not easily visible. Use a temp gun to the pipes to see.
***Check your total advance. It doesn't make a difference that you see 12 at idle. You don't drive at idle. If you are only advancing to 25 or 28 and your engine builder said "set it at 12 and forget it"....You might be owed a paint job.....By your engine builder.
I have seen many SPF cars and their factory paint is really good. I also have never seen one with the problems you are having. Forget the metal. That won't fix your problem.