Good stuff ftwelder.

So yes you have to make sure the BB is faced, but even the facing isn't perfect, and I read you can distort the shell slightly by clamping it too hard. 77 thou runoff at 58 cm isn't too bad, that's 0.2 degree off. At the saddle the runoff is bigger, but the butt-saddle contact region is quite large and one moves around.

Now your HT twist is a bit large I'd say. 37 thou on 7.5" means that the contact patch of the front wheel will be off by about 3.5 mm from the frame's centreline. The contact patch is probably around 2 cm wide for a road tire.

Btw I find HT twist quite difficult to avoid. I usually put my TT last so I'm thinking that having an accurately mitred TT tacked in place while brazing the DT/HT to BB would help? Or it's all in the brazing/welding technique.

Also I would not trust a fixture to give good alignment. It's better to tack in it, check for alignment, weld/braze outside the jig and check again.