You'll need to post good, high resolution pictures (or email them to me) of the following:
1. VIN
2. Engine pad stamp located on the front passenger side of the block
3. Transmission suffix / date code located on the driver side of the transmission toward the front
4. Rear axle suffix / date code stamped on the front of the passenger side axle tube
5. Rear axle date code cast into the center section
Without documentation it's impossible to determine a 68 Z28. You can do a "best guess" by the above, but you can never be absolutely positive.
Ed