I believe the sills were originally done using the fiberglass gelcoat resin with silver flake sparkles in it. That was popular in the 60's and 70's on dune buggys.
I could not figure out how to do that, so I sprayed the sills with rattle can (Rustoleum) silver glitter paint. It takes some practice not to get streaks. I then followed with 2 part Spraymax clear.
I think it looks close to original, but my car's original sills were quite dingy looking.