I'm planning to going with Catco universal fit cats but I'm not sure of what size to get. I think the stock size is 2 1/4in but i've heard people talk of using 3in converters,perhaps with headers(which i don't have)on a single cat car? I don't want to buy something too big for the stock y pipe but if i can go 3in, why not. I have a 3in pipe of unknown orgin after the cats and I might purchase a magnaflow.

 

Well if the pipe after the cat is 3in then go with a 3 in cat. You might want to change the y-pipe while you are ripping out pieces of the exhaust unless you plan on buying headers soon.