I'm curious, what kind of static thrust are you getting @ 7400 RPM w/ the 18 x 8 prop. That would give some indication what airplane would be suitable.
Also have you tried a velocity stack on the carb to see what if any benefit that might give to performance. I've noticed on many of my carbs, that the screw that holds the throttle butterfly to the shaft, the threaded end sticks way out into the venturi. I would think this might restrict some air flow. When I do my conv. I plan on grinding it back flush.