I bet it's either a 360 car, or perhaps a 318. They didn't make 340 Dusters in 1974. The guy who wrote the listing for the estate sale probably didn't know any better.
I guess it's possible the owner put a 340 in the car somewhere along the line...
I'd sure want some proof it's a 340. The fact that somebody other than the owner is listing it on an estate sale would concern me, as a potential purchaser. Does he really know what he's talking about?
I agree. The best way would be to give it the hairy eyeball yourself before making the purchase. But wouldn't you do that anyway given the nature of it?