put transmission back in my 97 mustang gt put the exhaust back in and on the passenger side rounded off the 18mm exhaust nut. about an inch gap between the h-pipe and the header sounds like a@# please help.

 

get a good pair of vise grips nice and tight on that nut, and spray the hell out of the thread with wd40.

 

You may try heating it up with the torch if you still have not got it. if you can get a chissle on the bolt head you can break it loose that way to.

 

If you just put it on then it shouldn't be that tight or locked on. Get a Bolt-Out set which will grab it and take it off then get a new nut, put on a dab of anti-sieze and don't go gorilla on it when you tighten it.I don't remember them being 18mm, just 15mm is the usual size, good luck.