I had the same problem on my '65. When I replaced the upper mast bearing it helped but it was still sloppy.
Upon investigation I found the lower bearing was gone (it is a ball bearing, the balls were gone

) and the shaft was severly worn, worn to the point of being scary! This post helps to explain the repair:
http://67-72chevytrucks.com/vboard/showthread.php?t=399139Hope this helps!