Does the car have original paint with stripes? Assume the rear end has been swapped, as you said the entire drivetrain has been replaced? As suggested, if it's a Norwood-built car (if the VIN contains an 'N' and the trim tag has 'NOR' on it) there is a chance that the rear seat bulkhead may have something written on it that could help ID it.
Otherwise, there are lots of clues that could add up to hint at a factory Z, but most if not all of those can be recreated now with repro parts, or by adding original used correct pieces.
Some clues would be a Muncie speedo cable firewall piercing, exhaust hanger reinforcement plate on driver's side rear frame rail, power brakes with front discs, multi-leaf springs, torch-cut shifter hole in the trans tunnel, in-dash tach (although not mandatory on an early Z), tire pressure decal, emission label, etc.
There is so much fakery going on that it is well worth spending time researching all the Z-specific stuff here and over at Team Camaro.
Plenty of folks here who will help if you can provide as much info as possible - good luck!
