The engine can be verified by reviewing the two-part engine block identification code, which is located on the front of the engine block at the base of the passenger-side cylinder head. The first part (Engine Block Vehicle Code), a nine character code, will always begin with "18" followed by an "N" or "L" then the last 6 digits of the VIN. The second part (Engine Production Code) is a seven-character code that always begins with a "V" and ends with "MO".
Transmission - A 1968 Z/28 came with one of three 4 speed transmissions - the M21 close ratio Muncie 4 speed (which was standard) or the M22 heavy-duty Muncie 4 speed (close and wide ratio which were optional). For identification purposes, an M22 will have a drain plug at the bottom of the transmission case. An M21 has no drain plug (must be drained and filled from a plug on the driver-side of the case). An M20 wide-ratio transmission was also available but was seldom ordered. This is ironic due to the fact that the low torque 302 performed much better at the start with a lower 1st gear.
The Transmission can be documented by viewing the passenger-side of the transmission case. You will find a five-character code (Transmission Assembly Date Code) that will always begin with a "P8" then followed by an alphabetic character and two numeric characters. Directly underneath this code you should find a 9 character code (Transmission Vehicle Code) that will match the Engine Block Vehicle Code ("18" followed by an "N" or "L" then the last 6 digits of the VIN).
Rear Axle - The standard rear axle ratio for 1968 Z/28 was 3.73 contained within a 12-bolt rear axle housing. The Rear Axle Date Code for the axle housing is located on the center section of the Axle near the inspection cover. The first character of the code will be an "A" through "L" followed by "1" through "31". The last character will always be a "7" or "8". The Rear Axle Ratio Code will always begin with two Alphabetic character (translates into an axle ratio) then several spaces followed by 4 numeric characters (designating month and day). The next character is a "G" followed by a "1" or "2" (designates the shift when it was built). If it is a Posi-traction Rear Axle, a one character alphabetic code ("D" or "E" or "W") is located below the Rear Axle Ratio Code.
(ripped off from
http://1968z28.com/document.html)