Bill, I wish I was already carving up the belly pan, so I could send a pic. There is a belly pan that is glued to the bottom of the wing, about 2 1/2" deep at it's shallowest, and it goes from l.e. to t.e of the wing. I will cutout the bottom, build in a half-round tunnel, and mount the pipe to the wing. The bottom will be open.

As long as you don't get into long pipe settings, it should be fine. The real problem with the 1.20 will be the short landing gear. Those will have to go if you run a 15" or larger prop.

The TT120RP would be a FINE choice for this airframe.
And it would be a great logical next-step to the Javelin.