Dropping the motor
Unfortunately from what I have seen you have to drop the motor to get the header off. If you can figure out how to remove the o2 sensor you should be able to wiggle it out without moving the engine.
Dropping the motor isn't as hard as it seems. Really all you have to do is rotate it about 3 inches forward.
If your interested in getting rid of the old header please let me know, I'm in need of a new one.