I understand he already has the Monster S4 R SSS swingarm and shock. apart from dismantling the 999 and measuring have no idea if casings are the same width.
S4R does have same engine as 999 as far as I know, so that should be straight swop for swingarm.
I would like to know if shock are the same length?
That's actually not true, the superbike cases are machined narrower at the swingarm pivot, so if he wanted to run the s*r swingarm, he'd have to be using the pivot axle from the monster, and have some custom spacers machined, additionally as already mentioned, he'd have to have the shock changed in some way. I believe that the superbike and monster shocks are at different locations transversely in the frame, so you'd actually have to modify the shock linkage pivot in the frame, as well as compensating for the different shock length.
Short answer is: it can be done, but you really have to want to do it, and it's definitely not bolt on.
That said, it's really not worth doing for the monster SSS, unless you were really going for the trellis look.
The older superbike swingarm, as mentioned earlier would be much easier, but is a worse swingarm than the s*r style, which is much worse than the 749/999 style swingers.
If you really wanted a SSS on a 999 the best thing would be trying to fit a 1098 swingarm.
Justin