Ah ha,,,,,welding now it makes sense.

I see now, yes for sure if your including a frame there would be alot of welding, and I see that shimming out the rails with epoxy etc will give you the alignment accuracy where needed.

Lot of interesting points in your post, I'll digest that some more later.

Out of interest why are you choosing to mount the rails on the side rather than on top? is there an advantage or is it to do with working width?

Also to reduce materials; at a cursory glance if you wanted side mount rails I wonder if you would gain strength and reduce flexing movements by putting those rails on the inside of the frame?
(Though no doubt you'd probably lose some working width).