Wow, some heavy-duty ideas there:)

I was also advised never to weld the thing 100% and i am not doing so now.

The base frame will hopefully be welded and gusseted/triangulated as one piece, right up to the support pads for the motion beams.

The beams for the Y-axis will then be bolted on so they can be adjusted, paralleled and shimmed level to each other and the bed area. Once they are set, the X-axis/gantry will be easy to fit.

That is my plan but my machine is far smaller at only 1250x1250mm cut area. I am thinking of using 50x50x5mm for the whole build including the single gantry beam.