Josh, from your drawing, I don't think any gussets are needed in those spots. You could, but as long as your welds are good and you have mentioned inside gusset plates, I don't really see the need. They might also complicate the build, especially the ones on top.

You say you are using 2 x 4 tubing, I would recommend 3/16 wall. With that you should never have any problems, especially if you tie your crossmembers into the sides well.


Don