What a cool find and should definitely be restored! I expected to see some sort of pop-top camper conversion but this is way cooler! And a V-8 T10 combo at that! I covet thy shifter! Not sure if there are patch panels available for that area. Might need to hit someone up for what you need off one of their parts rigs or possibly hand fab what's needed. A wood stump, sand bag and hammer(s) along with planishing hammer and or English wheel and some practice, you can make what you need. Weld in the new metal, and go slow - skip around your patches to keep from warping. Same when grinding welds. No pop-rivets, screws, fiberglass or the like. The problem will just return and get worse Only as much filler as needed to smooth it for primer/paint. Stay to less than a credit card thickness. Cut away all the rust and go from there!