Look in a book like Alan Colvin's Chevrolet by the Numbers for 1965-'67. It will tell you what every number means (part number, casting number, application numbers, date codes, axle codes, Trim tag decode as well as VIN decode).
There were only a limited number of options you could choose engine wise in a 1966 B-body and he lists each one so that you can figure out how your original engine was configured (carburetor size, horsepower rating, transmission type). Following the two letter application code was the last six numbers of your car's VIN making that engine specific to your car.
The other set of numbers tells you in which plant your motor was assembled and when down to the exact date.