In my experience, the bolts on the rear of the plenum and middle aren't likely to break. Most often it's the ones in the front, usually the front 2 that break. Over the couple dozen that I've done at work I've found the best way to keep them from breaking is to coat them well with penetrating oil as soon as you can, and get the bottom of the front two with little straw thing, only the front two are open and can be reached easily. I think the rear two are open from underneath as well, but I've yet to break one of those.
After that, get a torch, the hotter and finer the tip the better. I've used propane in a pinch on my personal truck, but oxy at work is great. Heat the bolts most and a little on the intake, you don't want to fry the head gasket, valve cover gasket or any plastic sensors you didn't remove from the intake, but don't worry to much about the intake gasket as you'll be replacing it anyway.after you've heated it a bit, doesn't have to even glow with heat, douse it with penetrating oil. Wash it off with a spray bottle of water so you don't light the engine on fire and heat them again. Do this 2-3 times for the front two and if you feel adventurous the next 2 or 4 back. You should already have the fuel rail and injectors completely removed at this point.
Once you've heated them and allowed them to cool, heat the heads on the front two bolts as well as the intake a little. A heat gun is preferable as you are far less likely to burn the heat gaskets. Then take your wrench and just put some medium force into TIGHTENING them, you don't want them to turn much at all, if they start to turn stop. You don't want to snap or stretch them either, you're just trying to break loose some rust. It doesn't make sense to me but it seems to work better to tighten first. Then put more force into loosening them. If they don't budge, go back to tightening. Keep alternating and applying a decent amount of force. A 3/8ths breaker bar works best as you don't have to keep switching the ratchet. Eventually they will start to move. Don't get greedy, if they have more than a hairs resistance stop after a little bit of turn and reverse direction. Just make small bits of progress. If it takes 5 minutes per bolt, that's less time to remove them all than trying to remove a single snapped bolt.
Feel free to add plenty of penetrating oil along the way, you cannot add too much.
If you do snap one, it's not the end of the world, you're just in for a pain in the ass. Every single one I've had snap has left me with enough to get a pair of vice grips on. Don't try the bolt extractors, I've never had one work on a bolt this size.
If it snaps, continue removing the rest and the intake will come off, you may have to force it, and if you do, try not to pry on the cracks along the edge, those are mating surfaces for the gaskets it'll just add more work later to clean them up. Once the intake is off, clean the heads of gasket material and spray them with water. You want the water to wet the head not bead up and run off. With the heads wet, take your torch and repeat the heating and drenching process from earlier, just use water. After a few cycles, heat the head, making sure to stop every note and then and put a couple drops next to the bolt, if the water boils stop heating, you won't cook the gasket at 212F,but above that I make no promises. Once you're ready get a pair of vice grips and grab as much of the bolt with as much force as you can physically get the vise grips to clamp. If you thing your hand will break, that's about right. Clamp them on from the side like you were using a wrench and make small back and forth moves trying to get the bolt to come loose. Don't get greedy, if you slip off or snap the bolt again, your chances of getting it out plumit. The bolt should come out eventually, it might take an hour, but it should come out.
If you just can't get it out, or it snaps flush with the head, you can try to drill out the first two, they will eventually just fall out, but the ones in the middle are blind holes and you may just end up damaging the heads,you have to be very careful and will likely need to drill them larger once the bolt is out and use spiral inserts to get the proper thread size and pitch. If you don't want to drill them out, a single bolt won't ruin anything and quality felpro gaskets should take care of it.
Good luck.
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