You need to find a good drill press, and simply drill the bolt head off with a drill that's about .010" larger than a 1/4-28 tap drill size. When the head breaks away the stock should come off easilly. Try a standard titanium coated colbalt drill bit running at about 200 rpm max. You don't want the drill bit to run fast as it willonly work harden the screw head if it's been hardened at all. Getting the rest of the screw out is pretty easy, but don't try to drill it! The screws are probably 316 stainless steel, and that's your first problem. Replace them with standard allen cap screws (Unbrako, KARR, or Allen brand), and don't buy the cheap Fastenall stuff (or Chinese). Before reinstalling the stock, run a new 1/4-28 tap thru the bolt holes, and the stainless steel will often gauld up the threads. Then coat them with a very light coat of Never Sieze with nickel. Reduce your torque spec by about 35% (I know they say 50%) and see how well that works. Never give the screws the full 40 in. lb torque with Never Sieze as you just don't need it.
gary