Aww, that sucks about the boxes. Oh well. I have thought that backing the FRP with some higher density foam would help stiffen it up as well as provide insulation.
Sounds like you've got it figured out. I agree that a thin sheet of high density foam insulation or the like would be a good idea. Sandwich it in between the inside and outside panels and you not only help yourself stay warm, but it should also cut down on panel movement (flexing, etc). I would say it might cut down on squeaks, but I doubt that'll matter beside the sound of Chevy thunder.
I've always wanted a gen 1 Trooper 2 door with round headlights. My ideal build would be a 4.3gm, 4l60 or 700r4, toy t-case,d44 front/rear and 10-33" tires.
Good luck and start a build thread when you get one.
Waggy Dana 44 Front: Aussie Locker, 4.56s
Isuzu 12bolt rear: ARB, 4.56s
Bumpers, 12k winch off the chevy so I can pull myself out of wet cement up to my roof.
Rock sliders, skids
2-3in lift, spring under in the front
3.4L 60* v6 from a camaro, isuzu trans and tcase