RE: How small a plane for a .40 engine? (reasonably)
I don't think you'll have a problem.
At worst angle mount the engine at about 45 degrees, head down, so that the muffler lines up with the fuselage centerline or thereabouts.
This puts the head out to the left leaving it partially exposed to the airflow.
Cut the cowl intakes for proper cooling.
You should have a blast with that combination. It should be a very good flyer.
Looks like a better deal than the Extra 300.