Getting this main frame square and level is important, everything else rides or is attached to it. I am often surprised but my unfinished floor is quite level. The upper right hand corner is low by 1/16", a couple of cut off discs for shims, one at the end and one in the middle leveled it right up.
I used two framing squares to square up the corners.
OK so I am ready to start welding. I'll post later today again, if I get it all welded up.