Don't go beefing up the motor box. It's plenty strong as it is. The only issue will be accessing the inside of the box to secure it in place after you have determined the correct distance out from the firewall to suit your engine.

Looking forward to your build thread. I had a lot of fun building this plane. I actually built the fuselage twice after ending up with a tweak in the tail of the first one I couldn't get rid of. One other item I just remembered doing was changing the bottom stringers on the plane from balsa to spruce to prevent hanger rash.