That's quite a neat idea there Robin, have never seen that applied yet.

I guess it depends on the intended use of the monster mill, spindle power and speed could dictate smaller (<10mm) cutters which would then indicate that smaller workpieces would be used - no use tickling a 300kg lump with a 3mm end mill I think.

That being said, a dual position gantry maybe offering the tool a 0 - 150mm and 150 - 300mm range could be useful, would need an assisted method of raising/lowering the gantry though - two trapezoidal leadscrews might work along with some heavy clamping method when in position.