Given that the shaft rocker system used is much easier on valve guides, with proper geometry, excessive guide wear doesn't need to be bandaided with a positive seal.  In your case, the umbrellas should work fine.  If you ever do a top end rebuild, that would be the time to machine for positive seals, if you desire. 

They act like an umbrella to keep the oil directly away from valve stem, like rain, they go up and down with valve and settle at top end of stem. You can have positive seal seals that clamp tight on guide but usually require machining, Ted or John M can be more descriptive. regards bill.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

I am in the process of changing the valve seals in my 1955 272. What is the correct position for the new seals, the old ones which where hard as a rock, where at the top of the shaft against the retainer. It seems like they should be at the bottom of the shaft against the valve guides which seem to be machined to accept them but there is really nothing to hold them there. Any suggestions? Randy