Have you given consideration to using steel tubes filled with epoxy granite? Cast iron is a very involved process and is not cost effective at all if you are only doing it once. It takes a long time to season the casting and machining adds more to the cost. It took rapidfire years to make his from beginning to end and he is doing it as a business.

You don't have to weld steel tubes, a carefully designed bolt + epoxy setup is just as strong as weldment and doesn't have residual stress problem to worry about. Just fill it up with non-shrinking cement if EG is too expensive. Read up on rapid machine design from MIT, I am using that as a basis to design my own vmc.