So lemme get this right, the original undercoating lasted 40+ years, and you wonder what to use?
I would either leave it alone, and apply another coat to spiff it up, or strip, prime paint and reapply. The factory didn't exactly put a heavy coat of paint under this stuff, knowing undercoating would be applied. Same with the inner surfaces of the fenders themselves. If you have the fenders off, you can certainly make it "better than new".