I have this plane but have not started yet . How about this .
Add ply reinforcement to the bottom of the current engine rails so they are thicker .Run this from front end to the first former , at leaset 1/2 thick . Now use a dremel tool , and burrow down a neat perfact slot for the clearance of the anti vibration mounts . This way you still have a good thick base to mount the engine but you also have a nice cavaity to allow for the anti vibration mounts .