These cars were built with horrible tolerances. So something that fits awesome on one car, especially headers, may be a tight fit or have clearance issues on a different one. Just the way things work out. Yes these headers fit a 74 Duster with a SB and PS. Not sure how many times that can be answered...
Watch out for the distributor cap getting damaged when jacking up the engine.
The other issue for the 73+ cars, shimming the spool motor mounts is a no go... which can make things tougher.
Generally they are a good fit. 73 K frame in this one.