I know the screws (PERSONALLY) to which you are referring..I recently dealt with a similar situation on a 1991 Doggedmini-van with a 3.0 L V-6.It only had one screw holding the two pump halves together and it did not want to come out.I started drilling with a reversible drill and left handed drill-bits and it came right out cool
As for the impact driver, try turning the screw tighter first as that was the last way it moved.THEN try to loosen it.You will want to have a good large vise to holt the pump housing in as you use the impact driver. BE CAREFUL not to hurt the pump housing in the vice!

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