What u can do if you cant change your BB is to go to the LBS and ask them for BB spacers, many MB cranksets come with them, buy one, probably they will give it to for free and put that where u thread the cog (where it needs to be it is pretty obvious), put the cog in, put the lockring... all set.

This will move the BB maybe 3 or 4 mms outwards...

Good luck.

ps: this is the simpliest way to fix it.