Some advice I read recently when looking into getting the exhaust manifold off was after leaving penetrating fluid soaking for 24 hours, start by tightening the nut/stud/bolt a fraction of a turn to break the corrosion before you start to undo. Helps reduce the risk of a snapped stud apparently, that would then need to be drilled out.
Mr Greedy This is my next project .Under my hood it looks nice other than the rusted exhaust manifold,, and nuts .I have everything i need to remove the manifold and then get it sandblasted . Also i have invested in an expensive manifold paint to finish it off .And it is a good idea to warm up the engine a little and then use your penetrating oil on the rusty nuts let it sit for 24 hours . And as you say to tighten the nuts up a bit first if you can . Looking at mine i think the studs will screw out before the nut comes off the studs.