[WILMASBOYL78]

This will be tough if the deck stamp has been milled down [assuming it is the factory motor]. The L78 would have a 3/8" fuel line and deep grove pulleys...beyond that you could check the casting date on the block and see if it lines up...check to see if it is a 4 bolt [look for the big plug near oil filter] check the heads to see if they are rectangular port versions and what date codes they have. The fact that it was originally a 4 spd car is one plus...the 68 L78 never came with an auto trans. Is the rear original?? if so, what gear ratio is stamped on the tube...the L78 would most likely have a 3.55 or steeper factory gear.

There are lots of Camaro gurus on this site who may have other ideas...