Had a couple of enquiries recently about the 28 swinger on the 21. Well, just done one for the first time and it's pretty easy.
We kept the 21 axle for ease of fitting so that we didn't have to mess about drilling and tapping the frame. and then modified the MC28 bushes, reducing the internal diameter (including the bearing distance spacer), and bunged it in with an MC21 shock.
Seems to work ok, but in my view it's purely a cosmetic thing because everyone knows that the Pro Arm's only really any good for wheel changes. I prefer the handling of the 21 swinger.
If you're looking for better handling from your 28, then I'd recommend modifying a 21 swinger to accept the larger diameter 28 axle and fitting that.
I'll probably get into trouble for saying that. Oh well.