Back when I drove a 96 chevy corsica 3.1 I went to Mr Z's Muffler.
http://www.sounddomain.com/ride/505751
They cut the collector at the bigget part and installer 2 1/2 in pipe a high flow
cat and a magnaflow muffler for close to $400. All the bends and welds were clean and it had a great sound. They did this in one day with an appointment.
I was really satisifed with their work and would recomend them to anyone.