I had similar problem on my 87 gw. Fortunately my repair area was small and I used 22 ga. sheet metal, rust-proofed it with POR-15 then some fiberglas over top. For a larger area use a heavier gauge steel,16-18 ga.,if you can weld it, great! Use some thin aluminium sheet metal to form a pattern that can be used to more easily form and shape the steel sheet metal outside of the vehicle before you weld it on.Then use POR-15 to treat the top and bottom to stop any future rust for good.
Dale Swaney<br />Warren, PA