Rear caliper clearence

I replaced all the pads on my 98 mustang and in keeping with my latest string of bad luck I couldn't fit the new pads in the rear. I used the right tool to screw in the piston but it only goes so far in then it just keeps turning without retracting. Because of this I could only fit the new inboard pad and the old(well worn) outboard pad.

This was the same for both sides and the pads are EBC greenstuff. Anybody else have this problem? Any hints?

Thanks,

Don