RE: PT20 Rebuild question, I goofed =\
Put your firewall in and use some triangle stock to reinforce it to the fuse side. I would then make a basswood shim under the mount and glue it to the firewall. If you have access to a belt sander I would make it to fit fully under the mount. If the mount is very stiff and solid say made of aluminum you might get away with washers as shims. Good luck in your repair.
Mark Shuman