I have read several places that the '75 280 came with 150hp. However, the plate under my hood says 170hp. What gives? Which is correct?

Both are correct. 170hp at the crank, 150hp at the rear wheels.

Aha! I was beginning to wonder if there was something screwey going on with my car (such as a previos owner installing a bogus plate or something). Good to know. Thanks.