Here is Prine's '74 360 Duster. The pictures with the motor on the chain show a flat flange on the DS manifold.
Prine listed his casting numbers
here. They don't exactly match, he shows 3614668 while the correct one would be 3614368, but I could see him thinking the 3 is a 6. I believe it is actually 3614368. Also in this thread is a '75 360 DS manifold, and it is not the same. Casting number is different, has a different shape and a cone outlet. So I would feel comfortable saying that after '74, the HP A-Bodies didn't get a large outlet manifold on the driver side.
Also, the earlier link for the for sale manifolds listed them as being off a 74 Dart Sport 360 HP and the DS manifold is a flat flange and matches the casting number.
In both cases, I would be surprised if someone had swapped the DS manifold for an earlier flat flange one, and changed the head pipe, rather than just swapped on headers. Not maybe people in that era would have even thought much about even saving the manifolds, let alone swapping one, rather than just putting headers on. My brother did just that, put a set of headers on his '71 340 Duster and junked the free flows.
Everything I have seen and heard has always agreed that the 3614368 flat flange HP manifold was used on 340 and 360 A-Bodies from 72-74.