Keep in mind, that if you're going to run a true dual exhaust setup on your car, that you're probably going to have some clearance issues between the left side exhaust pipe and the transmission crossmember, which will probably require either getting an aftermarket crossmember with a cutout for the left side exhaust pipe, or modifying your original crossmember to get some extra clearance for the left side exhaust pipe. As Ribbedroof has pointed out, GM never produced a '78-'88 A/G body car that had a true dual exhaust system from the factory, so they never made provision for an exhaust pipe running past the transmission on the left side of the car.
As for running an emission compliant system, you should be able to run dual aftermarket cats (check your local regulations first to be sure), that are either clamped or welded into the exhaust pipes. If you need to run a single type exhaust, you could use a system like what GM put on the later Monte SS/ Buick GN/
Hurst Olds/442. That would give you the look of dual exhausts, but be emission legal, and wouldn't require any changes to the transmission crossmember. You could either have the tailpipes exiting behind the rear wheels, or running all the way to the rear of the car.
Hope this helps. Good luck.