Hmmm, I do like the idea of starting with the gantry. We have some pretty large surface plates at work. I could make the gantry, get it all flat and level and square and use that and my machine level to shim up and align the Y axis rails. Getting the trunnion mounted would be the only tricky part, but I could use the same process to shim up the mounting plates and just indicate it all in with the existing axes.