You need a little information to start.. either the dealer name, or the area the car was shipped to. With that you can find (maybe) older dealer personnel in that area, who can tell you their ZONE number at the time your car was built. When GM changes the distribution point (ZONE number), all dealers in an area get their zone number changed along with it. These zone numbers can change fairly often at times. You need the Zone number for the area you are searching in for the timeframe your car was shipped.
If the original dealership was sold/closed/etc... then the DEALER number changes (and the old one goes away)!
Personnel in the PARTS departments are the ones I've found who generally KNOW what their Zone numbers are, and old personnel likely remember some of the Old Zone numbers associated with that area. For my car, the Zone numbers has changed 4 or 5 times over the years (each time GM/Chevy changed the location for their parts distribution warehouses).
Fortunately in my case, the Dealer Number is still the same, and I had other information pointing to that particular dealer which led me to find out all about how the Zone had changed over the years.
1) The Dealer number remained consistent as long as the same owner (or family) owned the dealership,
(but Dealerships got a different Dealer Number when ownership changed!).2) I'm not certain about this, but it's possible that Chevy/GM doesn't 'reissue' old Dealer numbers (I don't have enough data, even from the NCRS data base to test that theory). if this is true, then once that Dealer is gone, and after the Zone changes, and after 47 yrs, it's MOST likely the zone has changed multiple times, it's almost impossible to find the Dealer unless you have some independent information about your car...
