id take the mc18 arm off and put it up against the mc21 arm
use the chain run marks on the chain guide on both the arms to see where you are
obvious you need to use the 18 arm as a template
then you will know where to take any metal off the pivot end of the 21 swingarm
if you can do that spot on it should be just bolt up and done as long as the suspension
mounting points are the same on both bikes and 18 shock is same length as 21 eye to eye
they look pretty much the same just looking at them
hope this helps