The picture looks much beefier than the the model did. What I've worked with was 1" x 3" extrusions with slots that would take T nuts for 1/4-20 and 10-24. It would bow and flew just from leaning against it but was spanning up to 6'. We still use it for cabinets and control panel frames. I missed the part the OP posted that it would be for brazing, don't know if that causes the tubing to pull like welding does.