Well, Ive helped with a set of manifolds before with a local guy that went turbo. And I kinda picked up on a few things there. So when I started mine, I sprayed them down with some Amsoil Metal Protector (kinda like PB Blaster). I let that sit a little bit then I started backing the bolts out. When/If I got ot a point the bolts started offering resistance, like they would break or strip, I sprayed them down again, let that sit for a minute or two, then I tightened up the bolts back in like you were installing. Let that sit a minute or two and then I started backing them out again. All of them but one bolt came out after that process and on that one bolt I just had to do it again.
The same thing with the cross over bolts. The front side ones came out easy enough. The rear ones, I just used the above mentioned process and all was good. Now I havent done the rear manifold yet or collector, but I'll use the same process with them as well.