You really aren't going to see any difference in the two different setups on your car, ecspecially in the performance department. And your headers are really not a problem for our cars either. In fact, they flow extremely well and you won't see much of an improvement in performance until some sort of power adder is added.
If I were you, I would have an exhaust shop fab you up a intermiediate pipe, buy your own muffs, and get some tailpipes off a GT. I did just that on my car and it only costed a little over $200.
FYI: A 79-04 GT's exhaust will fit your car no problem.