Which K model was the original crank and which K model did your donor crank come from? The reason I ask is the thrust washer behind the crank primary drive gear changed from the early models to the later ones and the later ones were a bit thinner. It looks like you have the early (brass?) thrust washer on there in the first photo. Not sure if this would make a difference but it’s worth asking.
Also check the arm for the oil pump piston on the rear of the clutch basket. I believe it has an offset and there is a certain orientation for it. If installed incorrectly it can make the piston bind in the pump. Perhaps this is what is keeping you from sliding the clutch in far enough to get the retaining ring installed.