Don't forget to loosen up the large nut that holds the oil pickup tube to the oil pan and unscrew the nut that holds the pickup tube to the pump prior to pulling the oil pump down off the block. As you pull down, try to push the pickup tube forward and pull the pump back a little so the tube will pull out of the pump.
Be careful to pull out with the distributor shaft attached since these do not have a clip that holds the shaft to the distributor.
If any of the four bolts that hold the pump seem too easy when first loosening, it could be that the threads are stripped in the block from someone overtightening. If so, that can be fixed with drilling and tapping for a new threaded insert. DONT drill too deep. Only drill as deep as the depth of the new insert so it will bottom out on solid cast iron.
There is a round gasket that goes on the pump end of the pickup tube that should be replaced also while you have the pump off.