Won’t idle after new muffler

Midas put a new muffler on a p/u truck, it started, ran, coughed, died, cranked, wouldn't start.
They pulled timing cover to replace jumped chain, messed with it a coupla days. It'd start in morn , stall, no restart.
Frustrated customer called us, we got truck, checked it out, new muffler plugged shut .
No exhaust pressure from the tail pipe would be the first clue. Like a plugged converter. Or a tail pipe with a potato jammed in the end. LOL