I have recently found some broken manifolt bolts on my 99 Siearra. On the pasenger side I have one bolt broken, looks like enough to hopfully get some visegrips on, but the driverside has 2 broken bolts. Should I remove the bolts that secure the manifold to the Y pipe before proceeding with the other bolts on the manifold? Should the engine be hot when trying to remove the bolts? Can I use an easy out or will I have to drill out the hole and use a helicoil, or maybe a left hand drill bit? Looking for some success stories and pointers please.

I had the same trouble, and really after trying all you mentioned I found it easier to go to a muffler shop, they charged me 40 bucks to remove 4 bolts, well worth it, all of mine were broken below the surface, these guys put a fatty washer to the head at the bolt hole built a weld up to the washer then welded another bolt head to the side of the washer, they were done in less than an hour.
this did not include reinstalling the manifolds, I guess im cheap but i got them to let me just slap the manifolds back on quick like and drove the few blocks home to do the rest of the work, I installed headers since my manifolds were cracked as well. and as much as I can I like to be the only one who works on my truck.