I'd skip the flat plate mount and go simpler, go get yourself a stick of 10-24 threaded rod and a couple packets of 10-24 nuts. What you do is thread in the threaded rod to the backplate holes and stick the nuts down holding the back plate down then cut the rod to the length you would need to get through the firewall (use fender washers on both sides of the firewall if using this method). If one nut's thickness isn't enough to get past the lip on the backplate, use two... use a nyloc nut on the other side of the firewall. This will save you a couple ounces on the engine weight and is simple. If you still want to use the plate mount, get some nylon or aluminum spacers to go between the plate and backplate.