I am still running the stock 4.6 exhaust with the front cats having been replaced with pipe bent up to meed the shorty headers. There are only two major differenced between the 4.6 and 5.4 exhaust. The first is the location of the collector flanges and the second is the shape of the Y pipe. When money permits I will be replacing the headers and cats with a long tube and cat system. I have a Borla SIDO that exits before the rear wheels from the 1997 model year.