Which slant 6, Dusters.

I bought a 225 (or so I thought) I was told came from 74' Duster only to find out that they actually made a 198 in 70-74,to replace the 170 which I had heard of. I don't think the guy that sold it to me knew either, but I ended up with a great running very low mileage, engine.
/6 Matt, you said that they put 198's in Dusters. I wonder why it was singled out to the Duster? I know the EPA regs & fuel mileage reasons for the smaller engine and to replace the 170 but you could still get a 225 in a Dart, Valiant, Scamp etc. etc.
I wonder why the 198 was axed after 74' when the 225 went on until 86', or was it 87'?