If you are drag racing the car, you'll really want the direct connection ducktail style one. As the speed increases it adds down force to the rear. Do a search for the car called DustAAR to see a picture of it.
If you mean the spoiler looking one, I don't like it. A Duster I was considering had one on it... they don't seem like a factory job but back in the early 70's definately period look. I just prefer the go wing. Not like I will be spinning tires at 100 MPH anyways.