Hot rolled router frame

I'm still thinking about building a good size router. My 12X48 mill
is too small for some of the architectural projects I'd like to
tackle. I've found a firm in Miami (Smith Hamilton) with a large
blanchard that could grind the frame for a 60" (or larger) router. I
was considering a moving table design using bolted 2X6 heavy walled
hot rolled tubing for the table rails with linear slides. I've never
attempted anything precision on this kind of scale and would
appreciate your input:

1. Is common hot rolled tubing likely to be straight enough as it
comes from the manufacturer to use or do I need to order something
special? Is a 5' or 6' piece likely to be straight withing .020-.025"?
2. With a bolt together frame there are going to have to be some
weldments and holes added before grinding such as end plates and
bushings through the 2" width for bolts. Will the tube still be
straight enough after arc or mig welding to grind?

All input welcome... I prefer to learn from others experience rather
than my mistakes.

Roger S