From someone that has worked in a muffler shop, and lived in the rust belt all my life making my living for 50 years spinning wrenches. I have removed a ton of bolts from Pontiac exhaust manifolds, and any other brands over the years.
Heat the cast iron red hot and set the torque down on a impact wrench, or use a ratchet. If the bolt starts to squeak as it's coming out, stop and heat the cast iron up again, then after it's hot, try it again. You're most likely going to have to use an oxy acetylene torch to get enough heat to get it red hot.
The bolt in the picture will come out easily with heat. I've taken out much worse than that in one piece. All the worry about removing the old pipes before you try it, won't make any difference as long as you can get on the bolt heads. Pontiac engineers situated the bolts so they can be removed with the pipes on the car.
If the bolt head is undersized use a 14 MM socket on it, and of course a 6 point socket.
Drilling out grade 2 bolts that are made from cold roll steel isn't hard with a sharp drill. Cold roll bolts don't get hard no matter how many times it's heated and cooled. There isn't enough carbon content in them to get them to harden.
Be damn sure if you drill one out, that you get the drill in the center of the broken part, and follow the hole straight through. Getting the hole off center is going to mess the whole process up.
Penetrating oil on exhaust manifold bolts is pretty useless in my experience. heat causes the bolt to seize to the cast iron, and heat is the best way to free it up.
When you put it back together, use studs and brass manifold nuts, it will come apart easily the next time at room temperature, zero problems.
Having used cutting torches on many many exhaust systems over the years, I can burn a broken bolt out of cast iron manifolds. It takes some practice to learn to do it, but it can be done. When done, I can run a tap through the hole to remove the slag, and screw a bolt back into it......
An old timer showed me how to do this, saves a lot of time in an exhaust shop, when they don't come out 100% of the time.
