Severe backfiring

A friend has an 81 E series camper van with a 460. Has an air injection pump. It it backfiring severely through the muffler enough to have swelled the muffler and popped several spot welds. It looks dangerously close to a full rupture. Does this while decelerating. Muffler and plugs, wires, cap and rotor tune up done October along with winterizing. About 4,000 miles back had head gaskets replaced along with heads re-surfaced and carbon cleaning, etc. RV has been driven possibly 300 or less miles on three short trips to see this muffler nearly exploded. Engine smells as though running very rich. Does this sound like a carburetor loading problem or possibly that gulp valve? Thank you

Correct mileage is 88,425