I had a muffler shop build my 2 into 1 exhaust off the stock Y pipe, from the Y pipe I went 3" single into a 3" Magnaflow cat into a Flow 40 series 3" single inlet to a dual 2.5 outlet muffler dumped in front of the axle, Love it, cruising the parking lots it will set off alarms

This Y pipe from Summit might work for you.