I had a similar problem with the screws holding in the shut-off valve on one of my cars (other two no problems). Since I was replacing the valve I just ended up drilling off the head of the screw, taking off the valve and then turning out the threaded stud that was sticking out of the injection pump. I suspect you could do the same but be careful not to drill through the valve if you want to save it. I ended up ordering new screws from the dealer for that car.