as steve said before, grind whats left off or chisel em off, yank the header off so you actually have room to work, then carefully with a torch slowly heat around each bolt then while its hot give it a shot of pb blaster, the heat actually sucks the blaster into the threads then grab a good vise grips and just start slowly turning each one, itll let go eventually, you may have to repeat this several times before they let go but it beats drilling. this has worked very well for me with busted off manifold bolts. but first you need room. and dont heat it till its red! just get it hot.