I believe the '71 340 pass side exhaust manifold had a center outlet which means good chance exhaust won't clear. And once exhaust pipe is on it will be much more difficult to re-orient that 90 degree adapter. My two cents its simply not acceptable having it installed completely wrong and should be corrected while exhaust pipe is off. And tell shop you are not a mechanic and need them to correct the issue. They should have flagged that issue PRIOR to dropping in the engine being you said they are a mopar shop have a bunch of Dusters laying around etc....
