I always take the head off at the same time as they normal need a decoke when replacing cam . have you removed all the arms and compessed the valves down fully (pistons halfway down the boars not at tdc?)
I have doen this 4 or five times using a sykes and pickavant camshaft press, its a 30 min job. Make sure that you are lining the feet of the press exacty with the front and the back of the rocker arms. I take it that you have removed the plate at the distributer end of the cam that holds it in?
Right with the sykes and pickavant press you wound it down untill it hit the stops on the press, and with mine it did get very hard to turn when it got further down (due to pressure in the tappets), but if you wound it down till it was tight then left it for a few minutes i found that the pressure went out of the tappets and you could go a bit more (try doin a gradual half turn on each rocker arm then the next etc. and back to the start)