I just pulled this motor out of a 74 Z-28.
Block# 3935440 L-35
Heads# 3917215 Closed Chamber
Intake# 3883948 Q-jet
Last Numbers on vin for the car are 18R206833
It doesn't look like this motor has been rebuilt and the oil looked good with no metal when I drained the pan. The 68' SS it came out of also had a 4speed. If these Numbers match your SS you are in fat city.