Would I have any issues putting a dual exhaust system off an 84 cutlass on my 81 cutlass

 

Are you talking about a true dual exhaust system, or the factory "dual" system that was two mufflers with a "Y" pipe from a single cat? The factory system bolts on with zero issues. If you really mean true duals, your only issue will be the trans crossmember. You'll need a double hump crossmember.

 

No, basically the same car.