Thank you, Dave, for the compliments. It means a lot coming from the man that built a beautiful award winning teardrop.
Quack, I've read your posts and am a little confused as to what your plans are. May I suggest that you start a build thread, post pictures & drawings and ask questions there.
Tube bending: you could try cold bending the tube around a form such as a truck wheel of the diameter you want. You could try making a conduit like bender to your radius. But my best suggestion to get matching bends of all your arches would be to find a local machine shop/fabrication shop that has a ring bender with 3/4" square dies. Pay for an hour or two labor and be done.
Tubing: Why you are using 3/4" x 1/8" x 8' tubing? Find a local industrial steel/metals supplier. They should stock or be able to order extruded aluminum shapes in 20' lengths. Also sheet aluminum in a variety of types & sizes.
Rivets: I highly recommend using closed end aluminum structural rivets installed with a pneumatic rivet gun. You should be able to find them at a local industrial supply store.
Good luck, Fred