With really stubborn fastenings I MIG a tool, bolt or nut into place and work from there, the heat created does help but somethings you just dare not weld.
This is how it went with the exhaust manifold turbo flange.
I hope I'm not teaching you to suck eggs with this.
All tips are useful!
On my list of things to do this year (as well as a disertation lol) is to actually use a mig welder, ive done ARC but not yet MIG.
Will hopefully do a bit at xmas as my dads got one that he never uses!