A mate and I did a bit of shock stuffing around on the MC21 and vaguely our results were:

The CBR600F3 shock is 10mm(I think!) shorter, the resevoir tube is shorter than the NSR tube, and possibly need to shave 5-10mm off the width of the lower mount.

The CBR600F4i shock mount is the same length with the longer tube, but definitely needs the 5-10mm shaved off the wwidth of the lower mount.

I'll have to dig up our findings to make sure of this.