First off, how much is the finished plane going to weigh approximately? Are you sure that motor will do the job? The mount you have is for a stick-mount application.

What I would do is cut the existing glow engine mount flush with the firewall. Then select the appropriate outrunner based on the expected finished weight of the airplane. since this is a Cub, you don't need to overpower it. I'd say 100 Watts per pound would be plenty. Then you can determine what you need to do to mount the motor on the firewall and space it out as needed.