New floor pans are cut from 18 ga. steel, and plug welded to the flanges for the original floor.
Space the welds about 2" apart all the way around.
The roof is filled with a skin from a 78 Camaro, complete with the Camaro bracing. We cut cardboard templates of the roof, side to side, and front to rear, and took them to a salvage yard and fitted them to roof skins to find one with the right shape.
The bracing had to be bent to fit the existing flanges. The bracing is glued to the roof skin with structural adhesive.
The lower trunk flange was rusted, so I hammer-formed a new one on a maple plywood form, and curved it with a shrinker. The bottom flange was made separately. This was the only rust in the whole car.