Its when I've either switched or replaced engines with something different that I have done it. Remove old engine. Remove engine mount screws approximately halfway out. Tap on screw heads with hammer and drift punch (wood dowel) to knock the inside t-nuts loose. It may be necessary to relocate or remove the fuel tank to avoid injury to the tank. Once t-nuts are loosened, remove engine mount screws, engine mount, and fuel tank. Filll the mounting holes with the size larger hardwood dowel you can find. Re-drill hole to match .Use a strong two part epoxy and make the dowel length the same as the firewall thickness.

Start engine orientation and mounting process over.

Good luck!