exhaust leak between header and X-pipe

[mean81GT]

on my first stang i had a similar problem. i tightened the bolts down all the way, but it still leaked. After pulling the rest of my hair out, i realized the collector bolts had collars, and the nut was tightened right down to the collar, but the collector to h pipe flange was still loose. The headers i bought (FMS) came with bolts to match it up with the FMS h pipe. the flange on the h pipe is thicker, and therefore the collar on the bolts doesn't affect that setup. but i used a homemade h pipe, and the flange was thinner. i had to pick up shorter bolts. this may or may not be your problem, check it out.