2006mustanggtowner

alright so ive been getting a parts list together of parts im gunna buy and im stuck on my exhaust at the moment. i have mac cat backs now but wanna get headers and an xpipe or mac pro chamber. whats the process of replaceing the y pipe with an xpipe or pro chamber to put it to my stock cats or is there something that goes inplace of the cats. im just wondering because of inspection. also would all mac stuff be the best way to go being that i already have the cat backs. all help will be much appreciated thank you