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

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. If you are thinking of going with the later disk brakes/ big bolt pattern, download the 73 manual as well. Sometimes you have to do quite some reading. There's a fuel chapter(s) and emissions way back in the back.

These also outline the incredibly important 1/4" vent line for the fuel tank. Which originally is the EVAP/ carbon canister system.