I appreciate the help, many thanks!
It will be a week before I get back to the shop and put this into practice. On the question about the bracket holes. Each bracket carries a separate pin, so they don't have to be in perfect alignment. But for the use intended, the straighter the better. I will do a build post separately when I get started.
Now I think I will do one bracket at a time, rather than all in a row for fast production as I had originally thought. I will pre-heat the back of the weld with the torch, then clamp to a larger length of heavy tubing, then weld. I remembered I have a piece of 2x4x1/4 that will be perfect for this.
(As far as welding table goes, I spent my whole life welding in the dirt, or on a driveway if I was lucky. A couple years ago I upgraded myself to a Strong Hand light duty table. It is light years better than welding on the ground, and perfectly suited for 95% of what I do. It is no good for this job, but the heavy wall tubing will work.)
After welding each bracket, I will check straightness of the 2x2 tube, and use post-heat as needed.
Also I was originally planning to weld each bracket/gusset assembly to the tube at the same time. Now I think I will do the brackets first, and make sure everything is straight. And then weld the gussets.
I like the suggestion to minimize the heat going into the tube. Although with MIG I have got used to using as much heat as I can get away with.
So I will do the brackets one at a time, with much more prep than I had planned both before and after.
I am really glad I asked the question. That piece of 2x2 would probably have looked like a pretzel.