Welcome to the forum. There is some sort of bonus for transferring between accounts. Do you know someone who can transfer some orphan miles to you?
Are you planning any hotel stays or car rentals that might get you enough to put you over? Perhaps the purchase of the miles is the best option at this point.
ANC is a great use of miles - but depending on your itinerary and when you are going, it might be wise to wait to book until you can transfer those miles to DL. DL allows free stops within the US for award tickets (NW does not allow the stop on award tickets). So you can really maximize your award ticket by flying to say Anchorage and Barrow, King Salmon, or Kodiak for example. Two summers ago I used DL miles to fly to Anchorage and King Salmon (the jumping off point for bear viewing in Katmai National Park). Just Anchorage to King Salmon would normally cost $400 round trip so it is an excellent way to maximize your miles.
John