I understand your desire to stick with the original stock. When I head down one of these rabbit holes(AKA DIY gun stock alterations), I usually buy a takeoff stock on which to commit my atrocities. As it is, I second the suggestion of a screw extractor. I've used one a couple of times on buggered phillips head screws but it's a 'one time attempt'. If that fails, the next step is drilling the head off which is very tricky toward the end and will likely ruin the pad. That leaves you with the already mentioned cutting of the pad to allow use of ViseGrips-so maybe just sacrifice the pad and get it over with. If you're careful and are willing to use one of the slip-over pads, that will cover the damaged pad.