I believe that you should not need to shorten the right (shorter) driveshaft at all. I base my statement to the results I got after measuring the distance from center line (straight line from the middle of the propshaft mount) to the driveshafts CV mounting flanges. How it seems to me is that the haldex is 24mm wider than syncro (424mm vs 400mm) but only 6mm at right and 18mm on the left side!
After understanding this I would claim that the info Vegard was telling earlier concerning the Passat G60 front driveshaft would be appropriate since it is 22mm shorter than the original after 100mm CV modification -> close enough for the 18mm needed..
These are offcourse only stuff that I have measured with normal workshop tools and not 100% accurate and havent tested yeat but will be when I get my car on road. Why I'm posting this here is 'cause I would like to know how have you done this so far and how has it worked..