The rear frame rusting on the G Body cars from 1978 to 1988 is common. They are also prone to rusting of the radiator core support and the door bottoms. Another area on the El Camino of those years to inspect closely is under the rear window. Dirt and debris gets trapped under the lower window moulding and stays damp causing the problem. The lower rear quarter panels between the door and the wheel opening are another place that frequently rust through so check there as well. Paint blisters or small lumps in the paint in any of those body areas is a sure clue that the rusting has started.
If all that is wrong with that car ( truck ) is the rear of the frame behind the wheel arch and axle it might be worth having it repaired. The rear bumper needs to be from a 78 to 87 El Camino or any G Body station wagon of the same years ( Malibu, Buick Regal, Olds Cutlass, or Pontiac ). A sedan bumper doesn't have tail lights in it. New bumpers no longer exist from GM so its off to the bone yard or swap meet to locate one.
-- Mike.................................................... "Opportunities are spawned from crisis"