How old are your lifters?
Can they be pulled easily from the engine ?
If so maybe you need to either replace them... or take them out and clean them by putting them into a can of lacquer thinner one at a time and using a drill press or something to depress them several times.... to get the built up varnish dissolved and out...
then put into whatever oil you use and repeat before putting back into the engine....
You should be able to get the specs for how many pounds of force is supposed to be needed to depress them a certain distance... maybe you have a weak or broken spring in one....

Of course we have been avoiding the really obvious answer... switch to a 617 engine....Then you can enjoy setting the valves along with the rest of us....