Do you have any original documents for the car? If so, you can contact the original selling dealer (if he still exists) and start tracing forward to the person who sold you the car.
The other option is to go backwards starting with the seller and hoping that each person along the way is alive, locatable, remembers the car and has enough information to connect you to the next link in the chain.
Other options include DMV lookups (if you have requisite information and $$ for them) and posting ads in NCRS newsletters (Driveline) requesting information about the car.
I know some dealer invoices are available fo 77. Got mine through the GM media archives. The strange twist to it shocked me a little. The car was sold from a New Jersey dealer sill in buisness. And was ordered by a GM employee at Linden Assembly. I too worked there retired now. He worked one line over from me. The car was only driven for 12 yrs. The last inspection was 1989. I got it in Sept. 2005. took care of all the safety issues and drove it all last summer. I am getting ready to paint it now.
If it was ever registered in the state where you bought it, just contact the State Department of Motor Vehicles. Through a little paper work and a couple of fees, I got a complete vehicular title history report on my 76 and traced it completely back to when it came into the state. A little proding and they were able to tell me from what state it came from. Then off to that state, and did the same. I have names and addresses of each owner, and there is/was a lot of them on mine. Unfortunately, none of them live at the addresses still