I did something similar and ended up using some thin-jawed vice grips to hold onto the flange while backing out the bolt. Once the bolt is out, the riv-nut (as the industry calls them) can be ripped out with a good pair of pliers, some elbow grease, and 5 minutes. They're pretty soft and bendy without the bolt in them.
Dan
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2005 Silver CRD Limited

245/75R16 GoodYear Duratracs
Fumoto drain | ProVent CCV Filter
Stanadyne FM100 filter | Cummins fuel pump
GDE Eco | SEGR | BoulderBars | FrankenLift | Frankenskids