I think the 4s have 2.25 and the 6-8s have the 2.5
If the muffler youre thinking about is 2.5 it will work with your 2.25 the shop while cutting off your muffler will weld on a, lack of a better term, a flare to join the different sizes together.
And yup all it takes is lop of the old muffler and weld up the new one with some hangers and youre all set.
I have an extra muffler, universal bought a while ago. Its 2 3" tips I got off ebay, anyways never put it on so if you want it let me know, its the least i can do for the hold up for your parts.... you just pay shipping :P