Hi drunkn munky.
I'm almost done with that combination of 21 swingarm and wheel with the mc18 shock swap.
I tried a bunch of different combinations of the stock mc 18 and mc 21 linkage plates with the 18 shock... all ended up with having the ride height way too high. In the end I needed to make a custom plate, similar to what you have, to bring down the back end.
From my understanding, the 21 shock is taller than the 18 shock. My guess is with either of the stock linkages, you'll be way too high in the rear. You'll probably need another set of custom plates.
I don't have a 21 shock, but was keen on getting one from Tyga... so if you work it out can you post the dimensions for the new plate?