I don't think it is necessary to bond them together, and they will need to be attached to the tubing any way right? I think I would just drill and tap them so the tubing could be bolted to them. What are you planning to use for the bed ways? The original machine uses .25" x 3.00" CRS flat bar, and the bed needs to be bolted to the tubing also. Just thinking it may not be needed to solder/weld those.
On the drawings for the Gingery projects I would probably work on one machine at a time.