Hi, I think this design is coming apart at the seams and is now gravitating to "need some advice on a design for a bridge mill to machine steel" and not "do you think this or that would work" etc.

Designing on the fly always leads to redesign and yet more redesign, usually after the first metal is cut.

My advice.....for what it's worth.....is if you do not have design qualifications and are not aware of the pitfalls of bolted/welded fabricated design and subsequent machining problems arising, then get a design engineer to draw up the machine to your specifications and a draftsman to lay them out, then someone to make it......it costs less and saves a lot of headaches that way.
Ian.