[DougD]

You can use this one from Drake:
B6AZ-6766-B Mustang Scott Drake Oil Cap Chrome Push-On Style 1965-1966 | CJ Pony Parts

With this adaptor for twist-on applications:
C5AZ-6A868-A Mustang Scott Drake Oil Cap Fill Tube Adapter 1965-1973 | CJ Pony Parts

Once they're together, you can't tell there is an adaptor. It's a tight fit, with no rattling or loosening noted to date. They are both reproductions of original Ford parts. They were not original on a Cobra valve cover, but they look very nice. I'm using a pair of these with no PCV and they work fine. The chrome is high quality Drake stuff, and the caps have the FoMoCo logo in the middle for that vintage look.

DougD