Jim is correct: Install the valves and springs, then the lifter blocks. It is important to have the lifters themselves dry inside. When the lifters have oil in them, you cannot compress them and you won’t get them between the cam lobes and the ends of the valve stems.
In the early Motors Manuals, there is about a page of info about the Hydraulic lifters and procedures. They suggest using gasoline to clean the lifters of oil and dirt [hopefully not much dirt!] and installing the lifters dry.
Bill it is also very important to make sure the gasket between the lifter block and the engine block is positioned correctly and not blocking the oil ports. The gaskets have too small of holes for the oil ports, and as such if the gasket is just a bit off location, the oil ports can be blocked..