Cool advice on the full-scale drawing. Seems sensible to have a direct reference I can lay the work-piece on.
I like your wood block tube clamp.
My welding table is pretty flat. Not machine-flat, but I think flat enough for this sort of work. It's a tab-and-slot laser-cut table that shipped flat-pack. I clamped the whole thing together and "trued" it, checking it every way with a long straight-edge as I welded it up until I had a torsion box. It's been pretty reliable so far.
I like how much you appear to be doing with very little. I'm looking at my vise on the front-left corner of my workbench and thinking that I could easily have a setup much like yours. The only difference is that I don't have oxy-acetylene, so I intend to TIG-weld my tubesets. I'd love to try brazing if I can get my hands on a torch. It looks a little more forgiving than welding, but I could be mistaken.