Here is the link to a discussion on the radar I chose for my 2000' 3300SCR / http://www.maxumownersclub.com/forum...3279#post13279
My radar arch has glassed in marine plywood sheets that make up the back up plates for reinforcement underneath the three flat level peripheral hard point mounting places center left and right. I would think that all the 3300 arches would have the same construction techniques but then again there are probably no two 3300SCR's exactly the same. The way I mounted my radar is a little different that what you are proposing, mine is a an open array furuno that mounts directly to the top of the wing without a riser like you are talking about.. I simply removed the All around white light, and replaced it with two, on each side of the radar motor base. Instead of screwing into the plywood through the gelcoat, I made a 1/4" thk. stainless steel plate that acts as a threaded hole back up plate. it is very sturdy and does not flex a bit. The mounting bolts go through the wing and thread into the plate giving a bit more support.
Can you post a photo with the underside access panel removed so I can tell you if mine is the same as yours?
Anyway Hope I can help.
Bella Sera 3300SCR