<VV> So this was how Pontiacs, Olds and Buicks were found to have Chevy engines back in the 80's ?

[Ed Dowds]

The digit, that noted which car line the vehicle was made for, did not change according to what engine was installed.
The digit was for the body not the engine! 
In later years, after Corvairs were made, the VIN included a number or letter that indicated what engine was installed.
Starting in 1971 there was a code for the engine in the VIN.
My '88 Celebrity could have one of two engines: 173 V6, VIN code "W",  made by Chevy or a 2.5 4 cyl, VIN code "R", made by Pontiac. 
These codes are the 8th digit of the VIN.
Ed