how to tell if 70 Dart is real 340

VIN on the dash and radiator support match? It would seem unlikely that those would match on a fake.

I thought the washer was welded on the front facing side of the K-frame?

Has it got the wiring for the idle-stop solenoid?

I'm no expert on the slant six cars, but I don't know that they used different ball joints...torsion bars, yes, but ball joints?

I don't think any of the '70 340s came with AC, so that might be a dead giveaway.