I have a mc18 with a 21 wheel and swingarm crafted on using what looks to be a mc18 shock (shorter than a 21 shock). Ride height is fine in fact maybe a touch higher than stock but it does use what look to be custom linkage plates,

Im guessing these have been used to give more ride height due to the loss in height from going to a 17" wheel from an 18".
The 18 shock fitted is past its best although i have a decent 21sp shock but its longer, will fitting some stock 21 linkage plates drop the height back down enough to use the longer 21 shock?
Has anyone else got an 18 with a 21 arm that can provide any info on it?