When doing this swap, I always use one of these...
http://www.classicparts.com/1963-84-.../#.UOLSiazm7f0
It screws into the hole on the block. The z-bar slides onto it. I have seen one block in the past that had a flat spot where the pivot ball hole should be and i had to drill and tap it. This was a late model block though. All of the early ones I have ever seen had the hole already there and tapped for the ball to screw in to.