[Allen]

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Here's a few things for starters..
SS should have Power disc brakes. SS cars could have a 10 bolt rear, but If original BB, should have 12 bolt posi rear. Also, BB heater core, BB rad. support has larger opening, and the radiator Bolts to the support rather than the top plate method. BB frame mounts lift the engine up, and to the right. When looking at the nut on the air cleaner, it should be about an inch towards the pass. side of the car. Something else to look for, are the brackets on the rear studs of the hood grilles. [img]/ubbthreads/images/graemlins/scholar.gif[/img]

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To build on Dennis's list, even though it has a green interior, the steering wheel and column should be black and there will be an SS emblem on the horn pad.

There should also be a BB style heater box cover (like the Camaro) that has the heater core outlets closer to the fender, rather than right behind the motor.

Along with the 12-bolt rear end with the BB, there should also be a larger diameter fuel line (3/8"), and maybe two lines (a return line) if it was a '69 with the L34 396/350.

Finally, that setup would have came with factory dual exhaust, and there should be the hangers/mounts on both sides in the back.

Some of the items listed are SS equipment, and some are specific to a big block (heater box, radiator core support, engine mounts, etc).

The bottom line is: without any paperwork, documentation, verified past owner histories, or the original engine, there isn't an easy way to determine it. Mainly, you have to put together the clues.