A more reliable source than JCW is
http://www.alternatorparts.com , they tell you which they are giving you and they offer both negative and positive ground alternators in the 30 amp and 60 amp varieties.
I made my own 6 volt alternator using a self-exciting conversion regulator and I am quite happy with it. I used a Delco 10si and exchanged the pulley with the one on my generator. I mounted it by flipping the generator brackets around and it lined up perfectly on my 216.
