It really depends on the exhaust that you are trying to fit. The stock hangers on the body for the muffler, are different between the VR4 and the 1G. The VR4's muffler comes out straight or parralel to the gas tank, where the 1G is set on a slight angle wich is then compensated by the exhaust tips which make it appear to be straight.
I put a stock 1G cat back on to my GGSX, and it was a PITA. I figured it would be more straight forward, but realized after I got into it, that I would have been better off with bulilding a complete exhaust like I did on my VR4. Granted, It didnt take as long to "modify" the 1G exhaust as it did to build one from scratch, but it just tried my patience more.
Just my $.02