In addition to the above posts, I think it was also done to reduce engine vibration. If you will notice, most later-model cars have the accessories mounted very close. I have read this is done to reduce engine vibration. Accessories mounted far away tend to exxagerate engine vibrations.
As stated above, the alternator and A/C were switched to facilitate this. The heads were then requred to have bolt holes in them to allow the new brackets to bolt up. That is why some Chevy heads have bolt holes, and some don't.