Get your PC working and show Pic of what your meaning because without it not easy to visualise and could easily be talking at cross purposes.

Also at that size then I'd go with 5mm wall not 3mm. This will give enough material for drilling and tapping but more importantly help lower resonance from thicker wall. The difference 2mm makes is quite noticable on resonance.

Smaller tube size for bracing is fine provided there's enough of it and bracing in the right places. I'd go with 40x40x3