The 5 leaf springs and 12 bolt are a sign in the right direction.
The plate for the exhaust hanger is not unique to SS or Z28 cars. Basically any Camaro ordered with the N10 dual exhaust will have this, but still a good sign.
The power window/convertible wiring probably just means the auxilary power is used from the horn relay, via the firewall mounted circuit breaker that convertible power tops more commonly use. I don't know why he had to switch harnesses though. The A31/CO6 reproduction wiring and circuit breaker is readily available.
Front disc brakes?
Anyway, the 140mph speedo, big block heater box are easy bolt-ons and the body/firewalls are no different for any engine including 6 cylinders.. Not definite proof without the original engine I'm afraid.