The stock system was probably a y-pipe into the converter and back into two mufflers exiting out the rear with dual tailpipes.
You need to answer two questions. First, are you keeping the cat? Second, are you keeping the cast-iron exhaust manifolds?
If you dump the cat & go with long-tube headers, the Flowmaster american thunder system, the magnaflow system, and the house brand summit racing system will all work (depending on your budget & taste in noise levels). With the first two systems you can get an h-pipe & x-pipe respectively.
Most systems don't list that they fit anything newer than a 73 or 74 Camaro. That is due to the cat's. I've spoken with Flowmaster & they say, yup, they fit, but we didn't tell you that...
If you want to keep your manifolds or your cats, I would probably suggest not bothering... As it is, you'll see some improvement, but not a HUGE one (assuming you have a stockish engine).