Can you show a rendering of the rest of the machine?
Initial concerns are the 6 sets of holes for the rail. Linear rail manufacturers recommend milling a slot, or at least a shoulder, to align and straighten out the rails when mounting. If you are going to cast this thing, it seems that you want to make everything integral, when possible.
Also not clear about the center cross-member. It seems that it will limit your travel. That said, I am sure that renderings of the rest of the machine will clear that up.
That is a huge casting!