If its just the housing that's cracked, you can take all of your GS-R guts and put them into an LS housing. While you have all of your shafts and such out (which would be good to check to make sure all the clearances are stellar) it would be also good to drop in an ITR differential. Used ones can be picked up off of eBay for around 300-400 bucks. You can effectively kill two birds with one stone, and not lose the shorter GS-R gearing. I've got some pictures and such of the guts here:
Http://photos.yahoo.com/the_rooster_dont_die/

If you want more in depth info on swapping everything over, let me know.