if you can get some heat to it, that should help. Not a oxytorch unless you are very accurate with it's placement as to not damage nearby pieces, but perhaps a mapp or propane torch. The heat and resulting thermal expansion usually go a long way to breaking loose that corrosion from dissimilar metals. For sure use anti-sieze or as least grease when you put it back together.

Hopefully the long bolt through the alternator will still have a piece sticking out of the block you can heat up, then grab with locking pliers to twist out and replace.

Some times the best way to fix something like this is heat, and patience!

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95 E300 Diesel 265k and counting