I have the feeling that, unless you get the rigidity and bracing right, then sending it away for machining will only result in distortion of the frame whilst being handled for transport. Epoxy levelling seems to be a better way to get the machine bed level and far less costly than transport and machining. I would love to see any quotes you get for getting it machined flat as any company with adequate kit to do 3000 x 1500 is going to charge mega for a one-off.