Spray your manifold bolts with Deep Creep, or PB Blaster days before you want to attempt to remove them. Let it set for a day, and spray them again, repeat the process a few days in a row to let the penetrant work at breaking the rust loose. I've also read that you can heat them with a small propane torch and then take a crayon and touch it to the bolt closest to the engine, supposedly it will melt a little of the crayon and literally suck it into the threads of the bolt. I've never done that, but I have heard it works.
When you do start to loosen the bolts, only back them out a little at a time, then tighten them back down half the distance you loosened them. You want to repeat that process over and over, retightening them about half the distance that you've loosened them each time, slow and easy! You can also spray more penetrant on them if you feel it will help. Check out YouTube for some videos on this process, hopefully you can get them out without breaking them. Be sure to use a 6-point socket too, it's less likely to slip or round off the bolt head.
Here's one video I feel is a good one.
https://www.youtube.com/watch?v=9PKNCVXvZ6w