Drilling bolts totally out is a last resort imo. You're very liable to mess the threads up if drill to big/at angle. Def.. what rdub6 said. Spray bolts ahead of time w/ anything, Pb blaster sounds good, or starrett m1 spray, wd40. Let that soak overnight..
If channellocks wont fit, i assume they wont. But all you gotta do is break it loose, then work with it. If not, gonna have to drill hole and use a left handed tap. I forgot what its called. But some kind of screw/bolt extractor If you do it with a small tap, it will just break. Really, let someone else answer this that can solve your problem 1st try. Not me. lol.
Idea #2, Could also just pull the lower intake, and buy a new upper and lower stocker if its that much a PITA. But it should be doable with a screw bolt extractor if attacked properly. Research it (google), or wait for better input than i offer.