Rear cat gasket

Ok i changed this gasket prob 5 times now. The bolts keep falling out and then the gasket with it. Tryed double nutting, high temp locktight, metal lock nuts. They just rattle off. Well i decided to drill holes through the bolt and nut and cotter pin them... well go figure its leakin again. on of the cotter pins snaped.. What do i have left for options??? i was thinking welding the nut to the bolt