Is there a place where I can find an alternator already drilled out for the 7/16" bolt that goes into the head? The one I have I'm pretty sure will work but the hole is actually a little too big. Rather than screwing up another one I am hoping to find one that was already done correctly.

I've never had a problem with the bolt being loose in the hole. And i've used every type of alternator GM has put on a car/truck! Except for that monster cs-144.

You can get the correct mounting stud from most any of the suppliers.
(7/16" x 3/8")
OR
put a 3/8" helicoil in the hole and use a 3/8" bolt.