Ah - same problems I had a year ago. I too hammered and chiselled away, and sprayed copious WD40 into the joint, for 2 or 3 days …
Dispairing in the end I bought 2 rubber strap wrenches (1 for the exhaust the other to counter the torque on the front section) to twist them loose. Each side then was off in 10 - 15 minutes! Naturally make sure the WD40 doesn’t get under the rubber strap.
https://www.amazon.co.uk/Silverline-598531-Rubber-Wrenches-2-Piece/dp/B000O51BWW/I now have 2 old surplus exhausts, 1 with a conical end where I had been chiselling it!
