Does anyone know if there's a gasket, weather stripping, or something I can do to replace this nasty worn out "gasket" around the AC box, taking everything off of the firewall to clean and paint and would like to replace the stuff, thanks 1970 Cutlass Supreme

 

That's plain old rope caulk. Can be bought at hardware stores in the weatherstrip section, also used around windows. You have to remove the box from the firewall, clean up the firewall and box, and re-caulk in the recess in the box.
In case you haven't seen it before, it comes in boxes. It's caulk that is semi-rigid and in a string just wound up on itself. Not like silicone caulk in a tube.