If you are going to weld up the gantry from steel box section then you can use the epoxy levelling method to get a flat surface to fit the rails to in option A, but you should make it more solid. My own gantry is made from two pieces of 100x50 box section welded together to give a 200mmm high face for the rails to fit on. As always with a DIY build, your best option depends very much on what tools or other workshop facilities you have access to. Option A makes the Z-axis assembly much simpler to build as well.