Elrich11 I remember reading about your setup a while ago, looks fantastic. It wouldn't be too hard to attach the sleeve to a standard strut tube too as I'm sure I've seen done to a 510 in the U.S.
The only thing with the S13 gear is a lot of it is set up for drifting ie: rock hard (lots of poor quality Chinese dampers too) - so if the shock component is not high quality and readily re-buildable/re-valvable, it might not be the best way to go for a rally setup(?).
I do know that the Victorian 2WD rally class winner still runs S12 struts (built by MCA) in his S12 and not S13 struts as its believed the close to zero scrub radius works better with rally tyres on gravel. Interesting little point, though I think S13 setup would be better suited for tarmac track use if only just for brake options