Don't use inserts with an engine that size and a firwall that thin. Make up a 3/8 inch, or better yet, a 1/2 inch plywood motor mount using the aircraft grade plywood you can at the hobby shops. Oversize it 1/2 inch all around the motor box. Use T nuts to mount your engine. Epoxy and screw this to the motor box. Epoxy hardwood triangle around the perimeter of the motor box. If the motor box is finished, sand it off with 100 grit sandpaper. Use three hour epoxy for all of this. If you don't want to add the extra length to the motor box you can cut back the existing box the thickness of the plywood you are using and use the same process. Dan.