I'd definitely recommend a few rounds of a good amount of heat from a quality blow torch, then quickly cooled with something like
https://www.ebay.co.uk/itm/WURTH-ROS...item2cd367c6c9
Just in case they don't come out, here's what I bought that worked...
https://www.ebay.co.uk/itm/232570119279
You'll likely kill them all doing the job, but they're a lot cheaper than a strip down. Centre punch to mark the centre, then a starter dill bit if you have one... then the left handed set worked the way up in size, on a slow speed preferably with cutting fluid on the bit regularly too.
Then a tap of the right size to clean up the threads once you have them out.
Good luck.