If it's the closed type (about 4"+ thick) it could be cored or hollow (very unlikely it's solid). You'll want to bolt all the way through anyway to put on a backing plate. In any case, tapping with your knuckle or drilling a test hole will tell you what's in the middle. You can close up the hole with a little Quick Fix if needed.
Did you add the dinghy davits? On my 355 I used the previous owners weaver davits and added the weaver leaver which allows the outboard motor to stay on the dinghy. The motor rotates 90 deg while the dinghy is rotated vertically. The dinghy and motor do not move once in position, even in rough seas.
Thanks timjet. Are your davits bolted thru the swim platform? I am going to use 9" SS bolts with a 1/4" aluminum backing plate on bottom side of platform.
seldomdry The davits clips were installed by the PO and it looks like there were screwed into the swim platform. Not bolted or supported by a backing plate though I can't see to confirm that. I had to install a pad eye to support the motor support rod and that was simply screwed into the swim platform with probably inch and a half SS screws and plenty of caulk.
Be careful there is a fuel tank there, if you go into your aft bedroom you can take draw and back panel off to see what you have.