Well it's not as straightforward as one might think.

If you read the 700cc conversion thread that Kev did to his bike, you will see why, follow this link, please: http://www.xt660.com/showthread.php?t=5895

Whilst the Raptor part will slot in, there are some modifications that need to be carried out because some of the routings don't match, and if you don't have access to routers and accurate instruments in order for you to be able to tap holes and redrill others then your best bet is to get an XT top end and have it rebored.