Removing brushguard welded to frame

A cheap thin wrench wouldn't have helped - it would just spread.  The best move (AFTER soaking the nuts for a few days with penetrating oil) is to wedge a prybar between the frame & bumper to keep the square in the bumper (which is strong & sized correctly) as you back the nuts off.

During reassembly, use anti-seize on the threads.  Blue threadlocker is an effective anti-seize agent.  After a few days, spray the bolt's exposed threads with rubberized undercoating to protect them for years.

It took a couple weeks to return to the project, but I was able to grind the welds to get the frame cleaned up and flush again.   I'm not sure how pretty/polished it is, but the chrome bumper fits well.   Success!

Congrats!!!  The bumper does fit well.