Do you mean your trying to buy an extended swan neck? I’ve seen some silly long ones on 4×4’s with spare wheels on the back so I suppose you might be able to get, say, a Witter extended swan neck intended for a 4×4, for your standard witter receiver.
Also, you might find if you move the frame clamp/arm down one side of the hoop, and clamp the seat tube or downtube at a lower point, the bike will be more upright and hence more clearance for the bars. On my wifes bike we sometimes rotate the clamp so it clamps the top tube from underneath, this keeps the arm straighter and the bike further out. (just because its easier, I don’t normally use the rack on the van)