The joint is between the vertical tubes and the U shaped tube. I tested it on the bike, and in spite of the crummy welds it seems secure enough, with lots of bouncing and rocking etc. I guess the welds are not that highly stressed because of the geometry.
In the process of getting the tubes to fit, I ground away enough material so I needed to add the 1/4" feet on the bottom to get the length right.
Thanks for the help, I am sure I will be asking more questions on my next welding project.