My dh drives an '88 K-5 Blazer and I had the same worries about the fiberglass roof. First, a roll bar is a GREAT idea. It's the only way I will feel totally safe with the kids in the back of his Blazer. Second, at least in the Blazers, and I'm assuming in the Bronco's too, just ahead of where the roof comes off, there is more or less a built in roll bar, so the roof won't completly just crush. It adds extra re-enforcment to make up for the fiberglass roof.
I still worry about the roof being ripped off (saw it happen to my friend's 70's Land Cruiser when he was goofing off and flipped it at about 60MPH) and stuff hitting passengers in the back, but as far as anyone getting smushed, they should be ok. Esp for a kid in a 5pt harness. I would worry more about adults in the back, or even the front, being ejected, than I would about a kid in a Wizard. Also, I would assume the middle would still be the safest spot since it would be the farthest away from anything that could hit her.