OK here is the deal Ive recently removed the heater/ A/C box off my firewall to rebuild it and seal up a crack in the housing. My problem is this I can't find any company that sells a gasket/ seal kit for my heater A/C case any help would be app. Oh and its on a 71 Camaro small block of course.

 

i would just use silicon its just a plastic box that does nothing but hold heat in it.

 

To come close to a factory match use rope caulk. 3M makes it in either black or gray.
It will stay soft just like the stuff GM uses.