Unfortunately some of you dont have a good handle on what happens to what when "a little heat" is applied.
I would not have posted "the use of heat as a last resort" unless it worked.
Your choice - pull the box out and screw around or get under the car with a flat file and safety glasses and file two flats on the rounded head then wack your wrench on it and give the alloy case around the bolt head "a little heat" A small portable LP gas torch will do fine, even worth a try with you hot air gun if you can get the heat close enough because of the low temp of the heat gun. If you use an LP torch you can keep the flame small and only heat what is needed.
this will be heapes quicker than taking the box out and travelling all over the country to get it welded, then putting it back together...........
If your still soo desperate and want to get it welded, then drive it to an exhaust shop and get them to do it "in car" whilst on a hoist.