1965 Dart unknown hole.

Stumpy, OK, the reason its there on a V8 car is ALL the firewalls are stamped the same, Slant 6 or 273. Then if it was a standard Slant 6 it got a bracket that held that long linkage rod in place. If it was a 273 then it got that foam under the bracket posted in that picture above and the throttle CABLE was routed more left as the V8 was wider.

I don't have the link to our favorite Lady building that Hyperpak Valiant Wagon. She is using that hole with the linkage rod.
MASTER CYLINDER.jpg