With the cylinder heads in place, I’ve always removed the rocker shaft, then went down the line of cylinders, filling each in turn with 125 lbs. or so of air pressure to keep the valves closed… Some folks feed nylon rope into the cylinder, then turn engine by hand until that cylinders piston comes up and compresses the rope against the valves.. I’ve never tried that….
Br careful not to lose the two valve keepers after you compress the valve spring…..
Also, in my opinion, only the eight intake valves require valve seals.