Jeff, how did you adjust the spreader bar? Did you tighten it down a few turns after it was snug? with the fine thread and the high mounting position it doesn't take a lot of force to bend the frame and cause excessive stress. I set them by setting the suspension and tightening it until you see the numbers change on the alignment bench, if they change just a hair I tighten it down. It doesn't need a ton of preload, snugged up is just fine. It's going nowhere after that. I turned it out by hand, then locked it down. I didn't use a wrench at all.
I can't think of a gusset you could easily place there to help that area, only viable thing to do was to roll/bend a piece of angle iron to closely fit the contour of the spring pocket and weld that on. That should help. Also do check the inside of the spring pocket to see if the crack is visible there too.
That crack you have originates in the spring pocket, did they weld it there too? The metal is really thin and the stamped pieces have very sharp and sometimes rough/serrated edges, very easy for a crack to form.
