Raised fiberglass roofs aren't meant to be structural in and of themselves---the roll cages you mention are DOT required and necessary when/if the C-pillar is removed to accommodate taller rear doors. If that part is intact the roll cage is still a good idea but not required.
The problem is when so much of the upper factory lateral bracing is removed the distance between the B & C pillars is too great and side walls are considerably weakened. In the event of a side collision or roll over the fiberglass is an egg shell and exceptionally flimsy offering no support against those forces.
As for mounting things up there attaching them to the fiberglass isn't a huge issue but they do require some support other than the shell of the top. Without seeing the wood pieces you have its tough to say how much weight they can safely support.
Here's my own 2005 former wheel chair lift people mover with the tall rear doors, showing the roll cage DOT required for such a conversion. This type of structure would support a lot of weight but would require some additional support between the cage and actual fiberglass to tie it all together and hold solar panels etc against wind when driving. Have a look: https://imgur.com/a/kbyVF?grid
Feel free to ask any further questions.