Box section tube straightness

[EddyCurrent]

My frame was 3mm lower at one corner after welding, I designed it to have bolt on beams to support the rails, I managed to shim those beams to within 0.5mm of each other but as Neale says there was also very slight outward twist. I therefore used epoxy to level the top surfaces prior to fitting the rails. As Neil says, if you take the epoxy route you don't have to worry excessively over the frame, as long as it's strong and stiff.
That's what I've found anyway but it's everyone's choice to do what's best for them.