So, you have the standard bogie mounted couplings, and the dummy 2 pocket coupler block is still there on the buffer beam ? So no issue there for knuckle coupler attachment then ?
What you need now is a deeper multi height link and pin coupler mounting block. to modify and attach to the buffer beam with the two screws used for the existing dummy pocket. The obvious solution might be a Bachmann item
Coupler Housing (G 38 Ton/55 Ton Shay) [G811X-00Q01] - $4.76 : Bachmann Trains Online Store! similar to that used on their Shay locos modified to fit . A similar metal Accucraft item is out of stock in their shop. Any height disparity can be overcome with a cranked link to the lower height stock coupling. Which is a solution used on a 2 truck shay I have just bought. The only issue I can see is that the pocket would be unsupported lower down and that could lead to stress on the actual mount at the top. from forces exerted by the link low down.
I have seen some blocks made by some UK 16 mm rolling stock suppliers somewhere but for the life of me cannot remember who. Sorry I can't think of anything better right now. Max