Put 4 spacers between the firewall and the back of the motor. You can make these spacers from 3/4 inch dowel, drilled thru for the bolt. Of course your bolts will have to be long enough to go completely thru the lugs on the motor case all the way to the back of the firewall. This is pretty typical for gassers on airplanes. You can even buy aluminum spacers for just such a purpose.