The diamond shaped captive nuts are available from and of the G Body parts suppliers. Ecklers, OPGI, Mikes Monties, Dixie, etc. I used a 1” hole saw on the inboard side of the mounting “box”. Fished rod coaxed the broken parts out. Saved the “slug” and welded it back in when done reassembling. Another option is to cut a “flap” with a cut off wheel, bend it open for access, weld shut when done