I went through the whole disaster of that rear support. It's a very poor design, as far as rust is concerned. My solution was to use structural tubing instead of the aftermarket replacement because the replacement part seemed thinner than the stock part, and the stock part was inadequate in our salty winter climate. What you may not have noticed is that the entire weight of the rear of the wagon rests on that support. On each end are two body mounts where the support is bolted to the frame. For structural reasons it's important to fix that support before you instal the tailgate. That support holds the gas tank, tailgate, and the rear of the body and ties it all in with the frame.
I get into the details, with dimensions, starting at post #272, but if you go back a page or so for background info it might help you decide how to tackle this. You don't weld, but I think you could use some of my measurements and bolt things together to improve what you have there.
https://www.oldwillysforum.com/foru...roject-ran-when-parked-58-wagon.31171/page-14 Spoiler alert, jump to post #301 for picture of what I made to replace that flimsy support.