Why the different cuts on 2 and 4 and the ends of 3 not the same as the piece bolted to the wall ? You have one resting on top which makes sense as it will hold itself in place and one hung underneath so it needs support which doesn't make sense to me. Cut 3 same as the wall plate and level it up then drop the sides on it to weld them
Didn’t you start this thread by asking for help?
My only question is will i loose much/any strength jointing the box section like this,
Probably but not much compared to a properly prepped and welded mitered joint but certainly not enough to worry about.
if i need help i will shout up.