Back off all the valve adjustments, or loosen the rocker arm shaft stands until all the valves are closed.
Lock the flywheel with a couple of pairs of vise grips at the bottom of the bell housing and put air pressure into the cylinders one at a time.
Listen for air leaks at the carburetor, tailpipe, and oil fill cap.
Adjust the valves again, but this time, don't tighten the adjustments past the "zero slack" stage.
Start the engine. The valves will probably rattle a little, but the lifters will fill with oil so you can do a proper valve adjustment.
Then yank the hydraulic cam out, and throw it as far as you can. Install a Melling CCS-1 and solid lifters.
Jerry