I think the normal technique is to remove the push rod then remove the pushrod
tube and then remove the lifter.  Then replace with new lifter and use a new
expandable pushrod tube ($20 from Bus Depot).  I am planning on doing this on
my '84 GL which is also leaking oil big time from the tube seals.  Owners
manual says, "some intermittant lifter noise may be normal" or something like
that.
Also use a screwdriver (pointy end against pushrod tube and handle against
your ear) like a stethescope to find the tapping lifter.
Have fun and don't get any on ya.
Ken Wilford
John 3:16