Had the same problem years ago on a rebuilt that had sat for a while before being fired up. Pulling the pump is a big job, I would pull the distributer and "pre-oil" the engine first to see if the valve doesn't loosen up on its own first- it worked for me after only one filter.
Find an old distributer and pull the shaft out, take off the cam gear and then get a big heavy duty drill and put the shaft in it. Stick the shaft into the distributer hole and engage the oil pump then power up the drill. Do this a few times & hopefully it might free up the valve.
