Anyhow 40x 40x 3 box is all I genrally use lots of clamps on top and bottom rails.
If your using too thin a rail ie below 10 mm then look at stepping it up
Sequence the weld to alternate ends skipping every other one
Was trying to post a picture of 16 mm bar spindles between 40x 10 rails 2600 long and no greater than 1 mm out on finish done as described