What is your setup like? 3 vessel all grain, brew in a a bag? my initial reaction is you won't have room for everything to work well, (mashing, hop extraction.) If you really want to try it that way I would think using a little more of the specialty grains than necessary, use about half the base grain bill you would and use extract at the very end of the boil to hit your target SG. That way you should still get decent hop extraction and mashing is possible but still have enough in the way of OG to make a decent beer.