I can't help a lot since I have undercar exhaust but I do have the slip joint headers. One suggestion is to use something other than W-40. I would use a good, non-evaporating, penetrating
oil like PB Blaster or something. After that about all I know to do is tug, rotate and lever them around until they break loose and start to work their way apart. Rap them with a rubber mallet if you can get an angle on it. But I know you are kind of restricted by the clearance to the body opening as far as working them.
Maybe some of the side pipe guys will have some other ideas.