1999 Cadillac Deville water pump

My 1999 Cadillac Deville was leaking coolant from the bottom of the water pump housing. It has a Northstar Engine and uses a special removal tool to remove the pump. I bought a replacement pump which consists of a two piece movable inner vaned shaft within a fluted collar. I've managed to use the removal tool to expose the vanes of the old pump but can't get the fluted collar out of it's housing. How can I do this without stripping the inner housing threads?