I just found this listing on ebay. This is listed as the correct a/c bracket for the 455 HO engine. I have my doubts. Compare it to the attached photo which is the original bracket from my car. Note the cutout and the rolled edge for the heat riser tubing to pass over, which the ebay piece doesn't have.
http://cgi.ebay.com/ebaymotors/71-455-HO...1QQcmdZViewItem
By the way, I just re-examined the photos of your car from the auction. It is also missing the front a/c bracket that attaches to the two front intake bolts. (This at least is a standard piece from all Pontiac V8s and is cheap) I can look around in my parts and see if I have one.