Just a quick thought, instead of moving the gantry up to give you more room why not move the bed down??

By this i mean make a false bed which sits on top of your existing bed, with the false bed you would get say 0-150mm and without it 150-300mm. You have to remove it everytime you put a large workpiece in and it wouldn't give you the flexibilty to do the whole 0-300mm on one job.

It also would mean making your gantry sides a lot higher which would reduce the rigidity.

Personally i wouldn't bother though....!