Studs suck. Always.. If you can see your way clear to pulling the manifold, heating the studs up and either unscrewing them out or drilling them out do so. Then rethread the holes and put in good grade 8 bolts with neversieze or equivalent. You will never have an issue after that. Studs will always go to pot - metal is lowest grade and do not stand up. Done dozens that way over the years....
you know, I was wondering about this myself recently, and wondering why they'd use studs in an aluminum head. I can think of a couple of reasons:
1.) it's easier (safer) to thread a nut onto a steel stud than it is to try to thread a steel bolt into an aluminum block, i.e., less chance of damaging the threads. I ask you, what's most likely to fail, the threads on the bolt or the threads in your block? especially given some of these funky angles. I mean, how many times have you nearly cross threaded a spark plug even?
2.) sometimes access to nuts and bolt heads is limited, as on manifold lower fasteners. maybe not so much on the 22RE, but just a general consideration. sometimes, there's simply not the room to get a bolt in and started properly, let alone get a ratchet on it if it's tight.
head studs make me nervous, too, but I've found that when installing a new set, I like to chase the threads in the block, clean & dry thoroughly, and then install the new ones with some plain old blue loctite, and double-nut snug 'em up. I'm a big believer in the antiseize, as well, and am considering using the copper stuff on a current project since it's supposed to hold up much better than the aluminum stuff on exhaust components, however, I'm not sure I *don't* want those nuts sticking a bit. perhaps if it were something I checked on periodically. it's the corrosion on the ends of the studs and nuts that tends to stick and make the studs back out of the head.
but anyway, I've had decent luck with studs at a local hardware store here called Taylor's Do It Center. I'm not sure just how *national* these guys are. newer Lowe's stores have a great selection of hardware, as well, or you might try a fasteners shop, or even your local machine shop. of course, there's always online vendors if you're really stuck.
