Great looking jig! Since you asked for advice, I'll offer some input, but take it with a grain of salt - I am just an academic type.
I might describe this as a positioning jig, since it doesn't appear to clamp/fix any of the components in place. Depending on how much distortion you get, that may be just fine, but if you want more control over location, you might consider adding clamps in locations where you need things to stay put. You might also consider making the fixture tilt or rotate to allow welding access on the backsides in flatish position, assuming you are fulling welding the assembly while in the fixture. I noticed in this photo
there is some gap at one of the joints; you might consider how the fixture could be modified to snug those joints together while respecting the positions of mounting points. Lastly, it looks like there is not a left-right positioning feature for the main "hoop". You may want to add some stops at either end to ensure it is well centered.