If you are looking for super high accuracy then you will be facing the same problem as I have for setting up my lathe. and that is that even the bigest and best piece of precision ground steel will twist and defect if not bolted down evenly or to a flat surface. so it would seem it is more important to have a good solid base and if you torque everthing the same then it shoudnt matter if its Ali or steel (just my 2 penneth, I'm sure others will correct me if im wrong)

Have you thought about using a slab of granite worktop as the base and then building it in situ?