Try adjusting them with engine running. On a V8 it's pretty easy, I have never done a older inline so I can't tell you how to do it. What could be happening is you are adjusting the lash when the lifters are bleed down. Once the oil pressure comes up, the lifters fill and hold the valves open.
If you are having running problems with uneven idle or vacuum gauge all over the place, then the lash is to tight and you a few valves open. Its better to be to loose then to tight. When the lash is loose, you hear the clatter of the valve train just as you would a solid lifter engine, but the vacuum should be pretty steady on the gauge.
Is it possible to put the wrong lifters on a hydraulic cam? I would bet it has happened more then once, but someone with more knowledge of the older engines will have to tell you that.
To check what you have, run the engine at idle with the valve cover off for a short time, enough to fill the lifters with oil. Then shut it off and quickly take a pry bar and see if you can compress the lifter. It won't be much, but you should be able to tell if the lash gets wider. Don't compress the spring and valve, press the rocker arm and pushrod down. If you find it will not move, chances are you have solid lifters. It could take a few minutes for the oil to bleed out, so give a little time while compressing it.
Joe
