There are two seperate o-rings, in this particular area. One is located on the distributor shaft itself. The other is located under the distributor housing. There is a shallow groove that is machined into the underside of the housing that the o-ring is seated into. The housing can be removed without removing the head or the valve cover. There are three bolts that retain the distributor housing to the head. This housing actually covers the end of the cam, do to the fact that the cam protrudes out the back of the head. The end of the cam being a gear that drives the distributor.
I purchased both o-rings from the local Chevy dealer for just under $8.00 about three weeks ago.
Sincerely,
Nine Pine