I had the exact same problem with my 69 400 cvt, which had an replacement alternator (internally regulated) when I bought it. The dimple was rubbing on the valve cover, not much but it didn't look right. When I finally got hold of a numbers correct alternator for the car, I found out that it had a different orientation (12 o clock), even though I used the same belt. Was a while back but I do seem to recall that numbers correct alternator had a slightly larger pulley. Here's how it looked installed.
