I work on old tractors a lot. A trick a guy taught me was to first weld a flat washer to the broken bolt, then weld a bolt on. It works well.
The other thing I do with exhaust manifold bolts is try all the bolts and loosen the ones that come quite easily. Then get the engine as warm as possible before trying the hard ones. Sometimes a little heat before loosening, saves a lot of aggravation after.
2021 F-350 Platinum 4X4 PSD SRW 2016 Montana 3950FL (2) Honda EU2000i's
...and a few (twenty-some, but other than my wife, no one is counting) antique Allis Chalmers tractors