I had to do exactly the same thing - one of the studs was broken off nearly flush. You will have to remove the manifold and Y pipe. It's actually not that hard.. I just removed the tire and was able to pretty easily access everything. I had to use an EZ-Out to get mine as it was broken flush with the engine. Have to be carefully drilling - don't want to mess up the threads. I soaked mine really good with WD40 before even trying..came out fairly easy.
Also, recommendation: If the studs are broken, there is a good chance the manifold is warped. The constant heat, expansion / contracting, will cause the manifold to warp if one side is loose or not secure - it has room to "warp" as it constantly heats and cools. Depending on how bad it's warped, a machine shop can even it out - but frankly, I bought a new one from the dealer for around $60, which wasn't all that ridiculous. If it is even a little warped, it won't seal - and you really can't use any type of RTV sealant here, as it simply won't work. Make sure you install the gasket properly as well - one side has almost like a soft "felt like mateiral" while the other is metal... the new gaskets are a bit different than the originals.
I would also recommend buying all new bolts from the dealer - just replace them all. While other bolts will work (from your hardware store), the factory ones aren't expensive at all for a complete set, and they have the extra stud to hold the heat shield... using new bolts will ensure you get the right torque, etc.. And yes, make sure you use a torque wrench and set them exact - otherwise it won't properly seal... and you definitely don't want to over-tighten and strip anything.
I actually did both sides on my truck - the manifolds were BADLY warped. I set them on a flat surface and it would rock back and forth... IF your going through the effort of removing, and it has loose belts, just replace it - or carefully inspect to ensure zero warpage.
