Don't give up hope on Chevy records. At least some still exist. You have the GM of Canada records, the Yenko records, and the ZL1 vin numbers and info.
There may be more out there somewhere.
NICB has the original dealer and date for many cars.
Some Hurst Olds records were found.
I read about a guy who bought records from a Mustang assembly plant and can document some cars built at that plant.
There is the Pontiac Historical Service that was started by Jim Morrison who is listed as a moderator on this site
here is a link to PHS
http://www.gtasourcepage.com/PHSInfo.html
a link about Jim M
http://www.yearone.com/leader_ads/jimmattison.html
this is from Caddy web site
"Cadillac for the first time is releasing archive records for the purpose of funding the historical Cadillac Museum. Cadillac is now expanding the range of documentation offered for 1903-1970 models, to include copies of Salesman's Data Books, Operation Manuals, Optional Specifications, etc. Sorry, 1968 record sheets are NOT available. A list of available additional documents for the car will be included or may be requested by mail at no cost.
To receive a copy of the original Build Sheet and Historical Package for your Cadillac or LaSalle Motor Cars 1903-1970, Send $50.00 payable by check or money order, along with your vehicle identification number, year, body number, and style number of your Cadillac or LaSalle To:
http://www.caddydaddy.com/cadhistpage.htm