Federal emmision laws are simple -- nothing may be changed in any way shape or form regardless of where you live. Fortunately enforcement of that standard is absolutely zero. The Feds allow each state to enforce the regs on its own terms. Some states enforcement are more picky than others (Calif & Colorado come to mind)It is safe to say that most states will have as a minimum requirement if they check a vehicle that it should have all factory installed equipment in place. That would mean that if you went to a dual exhaust you should add a second cat. Do that & I think you should be ok in all but the pickiest of states. I would be leary of installing used cats. It is illegal for a shop to even sell them. Cheap universal ones can be had for around $75each anyways. Not worth hasseling with the unknown factor in used ones. Whatever you do good luck with your project
