"...there were no heads specific to the 428 HO...
Yeah that's where I was not sure. I don't think that any Grand Prixs got Ram Air heads or anything like that, but again I am not positive.
And who is to say that none of them were snuck off of the line with special goodies? If I worked on the line at the Potiac factory back then, and wanted a Grand Prix (instead of a GTO or Firebird), you can bet that the hottest motor that I could find would go into my GP. How about a 69 RAIII 400? To the best of my knowledge, that's hotter than a regular 428. But again, not sure...
As far as the value of your car, look at this auction. It's not the same, but if your car has all of the options that you mentioned, the values are probably close although if this one was in better shape it would cartainly be worth more.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewIte...rd=1&ssPageName=WDVW