1971 Duster exhaust smell in cabin after new muffler and tail pipe

70 Buster 340 said
Cut the 'gooseneck' off and put a chrome A body box tip on there.

That's what I would do.... to fix it.

Tail pipe will not be long enough for a chrome tip. I also believe he has a body plug missing to allow the exhaust into the car. A station wagon with an open back window is known for this. The reason some have a top deflector.