In addition to the barrel nut wrench, you'll need the action wrench too. You have to have some way to hold it still. Midway sells them, and they are about $50, but that's a one time investment.
I insert the front action bolt into the action, before tightening the action wrench. This keeps it from spinning a lot better than clamping the wrench down harder or even the recoil lug.
If you go to savageshooters.com, there's tons of hints in doing the swap, including step by step instructions.