What type of engine do you have??
A 5.7 litre will have true duals into the stock muffler then turns into a single.
A 5.0 litre will have the y pipe going into one cat and so on.
I have a 350 but took off the exhaust and install new pipe from the manifold back. For cats I bought two magnaflow cats. You can get an awsome deal from performance peddler. Search the e-bay store for the name. Real good prices.
After doing this all and keeping all 4 o2 sensors the ecm had no problems and no SES light came on.
Up here it doesn't matter but that's what I'd do if I were you. If you are limited by laws then I guess you would have to find an aftermarket y pipe.