Does anyone know what goes in this spot on the firewall?

67Dart273 said
OP do you not have a factory service manual? FACTORY, not Haynes or Clymers. Run over to MyMopar.com and download one, free. SOME OF THOSE GOT THERE because of the guys right here, and the 72 manuals is one of those.

Yes to this. The site has a 1972 Chrysler factory manual - virtually all of the Valiant info is relevant for Darts.