My wheel wells are down to bare metal because new quarters were put on and car was sandblasted. What would look most like the original undercoating. Just regular spray can rubberized undercoating? this is a 68 Cutlass.

 

The guy doing my car used spray in truck bed liner. Says it is an almost perfect mimic. I haven't seen it yet to confirm.

 

If looking to go the original route and it's down to bare metal under there, make sure you either etch prime or epoxy prime and paint before any kind of undercoat...#1- the factory actually painted the rear wheel wells, #2-you'll need to protect the metal from rusting....IMO I would use a good quality rocker shultz in there. It's very durable and offers superior stone protection to rubberized.