Here is a copy of the one I built and used. I had it for sale but is now sold. Worked great. I also mounted to the hinge frame and striker area.
Ron
That is one fantastic rotisserie and whomever bought it from you got a great deal, but unless you plan on restoring multiple car/trucks (in my opinion), Kevin’s (Klhansen) is much more compact, simplistic, probably cheaper and easier to build.
You also can’t go wrong with 52carl’s idea with bolting 2x4 to the firewall. We just need clarification on the details and maybe a photo would be great.