Thanks guys for the time,
The measurements were accurate, and no welds were present indicating it had not been patched before. So after measuring it's not the frame as mentioned by others above. I was able to put grade 8 bolts through the frame which was a pain. I wound up ordering an offset wrench off amazon as its the only way to fit a wrench through the access hole and behind the nuts ( would love any tips y'all have in getting a wrench inside the frame to tighten the bolts)
Starting frame side first, I was then able to get a bolt and nut on the bumper side attachment point. Tightening this eventually snugged itself all together, though I'm still not sure where the error lay. The brackets were square as they fit perfectly in my carpenter square, and the frame measurements were spot on as listed above. Anyways, in the end the bumper is on and all is well. Thanks guys, will post a pic tomorrow.