Here's another post of how to do it.
http://986forum.com/forums/boxster-general-discussions/72900-coolant-tank-replacement-post-replacement-observations.html
Like Clickman I removed the hoses in the engine compartment from the metal lines instead of the tank manifold.
I also used screw down clamps for easy of installation. I know some don't like it, but I'm not here to debate it. Additionally my cheapish clamp pliers broke from squeezing too hard on a clamp.
I don't think having the car in the air would help. I did all the work from the top side.