Welcome to the CRG -
Some other quick clues:
Four leaf rear springs (if original springs). Newer appearing springs obviously trump this -
12 bolt rear dated to the car (could always be replaced or sourced, but dated rears don't grow on trees).
Muncie speedo cable location through firewall, plus a manual trans reverse harness (can be added, but some fakers don't remember all the time)
Rear bumper guards, or the holes through the rear panel if guards are missing. (can obviously be added, but people sometimes forget).
Gauges and tachs don't always indicate a Z, but if everything is there and dated to the car build, it just can add some possibilities.
We've had many discussions about '68's before: all of the above indicators can be added or faked - nothing works better than original paperwork from the State or previous owners (some of which can also be suspect). Good luck, and post what you can to let us shower you with opinions -
Regards,
Steve