I'd highly recommend jetBlue. We've done the same thing as you for the past two years, using WN Rapid Rewards points to get to SEA or PDX, and then flying jetBlue from SEA or PDX to ANC. For the past two years jetBlue has had flights in early June and late August-September for $78, or around 4000 points. If you buy early enough you can also get flights for around $99 each way the rest of the year.
There has been an offer for 40,000 jetBlue miles. I'm not sure if it's still around, but that's what I'd recommend. Delta also regularly has flights for 7,500 miles from SEA to ANC and FAI, so that may be an option.