I wouldn't whack the screws, even with a 2x4 over them.
What I've done in the past, and it's the method recommended by John Walker, is to stand a couple of 2x4s on the 2" side (so the 4" side is up). Then put some towels or rags down between the 2x4s. Space the 2x4s so they're wider than the alternator, but not as wide as the entire housing. Turn the housing so the alternator is facing down, and whack the entire housing on the 2x4s. After one or two blows, the alternator will drop out of the housing without damaging the screws.
YMMV. Good luck!
