[Bennett548]

Adam is right.

Fasteners work best when they generate clamping forces high enough that the friction between the two parts being fastened is enough to keep the joint from moving. The bolt or rivet is never in danger of shearing or loosening because there is no movement in the joint.

That is why you should either put an aluminum spacer inside the tube, or figure out how to fasten the gusset plate to the tube wall itself. This is normally achieved with blind rivets. Rivnuts or small self tapping screws will also work.