Is the stock filled with some type of soft foam?
The black stuff (hard rubber or plastic?) to be removed looks like 1/4 inch thick but not hard and might be used to hold screw inserts inside a foam filled stock. Removal by sanding might be slow & messy with lots of fine flying junk. A coarse wood rasp could be used to get most of it removed. Be careful not to damage any edges and finishing the job by hand sanding to get a nice flat surface. Inserts for screws would be needed for a foam filled stock but would need to be big enough, possibly 1/2 dowels, and well attached. If a good bond could not be made with the foam, the inserts would need to contact the inner stock shell surface. I like JB Weld.
I like T25 head, 1 1/4 inch, wood screws and they may be driven in without damaging the outer recoil pad surface and the T25 head is better than Phillips or slotted head for ease of driving.
I would try to avoid attaching the new pad to the stock with epoxy because you might want to remove it easily. I have installed pads, including preparing the stock end surface, using a large disc (10-12 inch diameter) sander. These are sort of pricy and would need lots of use to justify their cost.