also if your not near any fuel lines, i have used a torch and vise grips but if your space is limited i guess that wouldnt work.. i just had to hacksaw off a bolt on my 84 camaro's y pipe it was double nutted on the bottom , the bolt jiggled around but wouldnt come undone and i didnt have any help availible to hold the top wrench so i just hacksawed it off, took along time but if you got a fresh blade it shouldnt be bad, just go slow when you get close to the end so you dont cut your y pipe too.
a friend of mine as the easy bolt-out bits, that generally accept an 1/8" range of diameters, one might be 1/2-5/8", for example. They're like an external easy-out...when you twist left they dig into the bolt. I bet they'd be ideal for manifold bolts. Here's a Sears listing for clarification.