Last Thursday I returned some parts from the wreck and had some discussions on this exhaust. His suggestion was that likely it isn't welded, just totally rusted to bits. His advice was to use a pneumatic hammer with a sharp chissel and get some force between the flanges.
So that's what I did today and after some careful and later less careful hammering they did indeed separate! So tip for anyone who has to do the same job. One leftover part of a bolt remained on the manifold side. Once I had this one broken off I could tap the last bit out.
The remaining job was to get the O2 sensor removed, but that went quite well after getting enough heat in it with a torch. Now I just need to file the slightly damaged edge of the manifold flange while I wait for the new cat to arrive. Overall with this little tip it was a lot less work than I was afraid off
