2nd July 2011 - Did not go exactly to plan. Pulled out the plugs, off with the rocker cover, removed one adjuster screw, went to fit the silencer, would not fit. Then I remembered that my Bedford truck valves have oversize stems. So I was not able to fit the valve top silencers, but was able to fit the spring silencers to each end of the rocker arm shafts.
Marvel Tappet silencers put away for the future when and if I change the head over to one with standard valves.
No great drama, good excuse to reset the tappets, and a couple were a bit loose.
End result is tappets are a bit quieter, but as a lot of Chev 4 drivers say "If you can't here them their too tight"
Did not need to clean the plugs. Those Autolite 3077's are great.
The easy part of the whole operation was resetting the tappet clearances. This has always been a job that I never liked doing, very fiddly. However a while back I purchased on EBay a vintage tappet adjuster, made by Snap On Tools. This is a combination socket and screw driver in one, and is excellent for a one handed operation, while you have the feeler gauge in the other hand.
See pictures below.