Something that has not been mentioned (but has come up before over the years) is that some auto parts houses will try to unload their smaller units on you. I think the standard C4 is a 105 amp unit and ours is a 120 amp. I ran into this so I simply bought a rebuild kit and kept the original 120 amp diode pack that came in the original one.
The link to the rebuild is on TechNet at:
http://www.zr1netregistry.com/Inform...r-Rebuild.aspx
Another difference between alternators is that ours has a threaded mount hole on the back for the stock mini bracket that mounts to the head. The smaller non ZR-1 units don't have this hole and therefore the bracket is useless. I suppose the good news is that without that bracket you can remove the alternator without removing the plenum. There is a TechNet article on that too.