You could check the front stamping pad located on the front of the engine just under the passenger side head. On high performance engines, part of the VIN is stamped there. That's what is known as a "numbers matching" car.
They didn't stamp the low horse motors like 283's however. To tell if it's an original without the VIN stamp, your basically looking at comparing build date codes.
Parts like the block, heads, intake, etc will all have a cast date. This date needs to pre-date the cars build date (located on the cowl tag) by about 2 weeks roughly. Naturally if these dates are "after" the car was built, that will tell you they couldn't be original. Or if there is a large difference, it also can indicate not original to the car.
The cowl tag is located on the top of the cowl near the passenger side hood hinge.