Mike,
I would avoid mounting things to the cowl for maintenance reasons. You'll have to detach those things every time you take the cowl off. The most clever mounting location that I've seen is tucked into the space between the fuse an cowl. You might also consider a maintenance hatch on the fuse. I've seen many scale modelers do that. The dummy radial is only styrene plastic. It would probably crack around the stuff mounted on it.

I've flown with 15x12, 17x8 and 18x6 props. The 15" prop had plenty of pull, so I wouldn't be concerned about smaller props on the Gee Bee. I'd recommend a prop in the mid-range of the possible props for your engine for a good balance between speed and braking action. I've flown the Real flight Gee Bee and thought that is is pretty accurate. Practice a bunch of landings and you should be well prepared to fly the real GP Gee Bee. My Saito 180 has a collet with grub screw on the needle valve end that I fastened a thin, flexible piece of piano wire into. The piano wire is then threaded through a hole drilled in the firewall and fuse an inch from the corner were they meet. It then exists from the gap between the fuse and cowl at about a 45 degree aft angle.