I went through this nightmare recently after the alernator died on my '84 TD. The bottom bolt was seized in the two-inch hole in the aluminum bracket. Afraid of snapping the bolt, I used assorted prybars and a 2x4 to move the alternator as needed. First I removed the top adjuster bracket. Then I pried the alternator out as far as I could to allow room for my socket to unbolt the alternator bracket from the engine. Then I removed the alternator, with bracket, so I could work on that bolt someplace more convenient, with a breaker-bar, electric impact-gun, and liberal doses of oil, until (two hours later!) it finally came out. During this ordeal, the bolt head did get a bit rounded, but there still was enough left to be able to tighten it with a six-point socket. Plus it's now well lubed, so that it should be OK for awhile.
You migh try soaking it with some type of penetrating-oil for a day or two before attempting what I went through. Good Luck.

Happy Motoring, Mark

__________________
DrDKW