For my front strut mounts I just ran the self tapping screws thru the fiberglass and into the outer frame flange on the sides. And thru the front pan for the center.
For the rear, I could not determine what was under the belly cover without removing it. So I made a cross brace from 2"x2" square tubing cut at a 45 degree angle on each end and two clearance holes drilled in each end. And screwed that directly to the frame thru the fiberglass. A metal shop could easily make a cross member like that for you and simplify the installation.
