I had a problem getting my intake manifold off, too, for the exact reasons you're having problems. If no one else helps you, you can still do the rear plugs, it'll just be a lot harder

If you can't easily get the manifold off, you can just remove the windshield cowl and that'll give you some room to work with. It'll be a bastard squeezing your hands and tools between the manifold and the engine, but I was able to do it, and I have huge hands.
Just a suggestion, though.