Id try to clean up the under carriage by best means...maybe if you can cover up fragile parts like hoses etc, a light sand blast or bead blast...then either POR it under or "rustproof" it lightly with spray on... or some 'oily' substance....back in the 60s lots of cars in Europe got a light oil spray under to keep rust out...usually at oil changes.. ...I wouldnt start a frame off on that car.