GM's Chevrolet motor division rose to prominence by offering the quality of a Cadillac at a Chevy price (entry level; aka cheap). To accomplish this GM used statistical quality control techniques that they learned in building equipment for the US government during WWII. To identify what parts were used to assemble cars every part was numbered and date coded (if it was purchased from an outside vendor that source was also included).
Every part had to pass inspection whether made by GM or someone else. In the case of machined parts (such as engines and transmissions every part was 100% inspected with go no go gauges. With purchased parts a sample was selected (size of the sample determines to what level of certainty you wanted to achieve). That big "T" on the bottom of the part stood for the Tonawanda, NY
foundry and assembly plant that cast and made every big block Chevy engine. Also on that part will be cast a date code and a little clock showing what shift it was cast. This way any faulty parts can be traced back to the person that was involved with the making of everything that went into your car.
All of these detailed records (it took the first and biggest computers made by IBM to keep track of all of this using Admiral Dr. Grace Hopper's contribution to the business world, her programing language called COBOL) to keep track of everything. These records were vast and took up a lot of room. Warehouses were dedicated to the storage of these records (at least for the seven years that the IRS required) and then they were hauled off to the city of Detroit's incinerator to be burned. This is why you can not get any information on your car unless it was manufactured in Canada, because Canadian law dictated that those records be preserved.
A copy of the computer generated build sheet may have survived by being hidden somewhere in the car but that is a indication of someone not following the rules by not throwing away the trash. It raises the question of what else did they (the union worker putting the car together) fail to do.
You can buy a book such as Alan Colvin's series of books Chevrolet by the Numbers
which is considered to be the definitive book series on the subject of decoding casting or part numbers.
Or this little pocket sized hand book written by Mark S. Allen's Chevy Impala Factory Part and Casting Number Guide 1958-70:
For just the big block engine if you are shopping in the junkyard this hand book is helpful: Chevy BIG Block Factory Engine Identification Code Guide
I also find his hand book helpful for deciphering cowl tags while in the junk yard as well:
If you are going to mix and match a BBC motor using Chevy factory parts this book gives you guidance Chevrolet Big Block Parts Interchange Manual
If you are looking for car parts then this book can help Chevrolet Parts Interchange Manual, 1959-1970