I installed my manifold when the head was off the block. if its giving you that much trouble, get a head gasket set and pull the head. I started at 8am, had the head off by 11, took it to my friends shop, machined it, and had it back on and running by 8pm lol. When u have the manifold off make SURE and change all the water lines that go on the manifold, between the head and manifold, and between the manifold and block. TRUST me itll save u time later because replacing one of them on the car when it split open and dumped hot coolant everywhere it a real pain in the ass.
Andy