I'm not sure about a 60 or 60+. For a more sure bet for that, I'd try the Kobuk or Koyukuk. Maybe the draw for the Martin River/Bering River. I just don't think there are that many 60" bulls in the Girdwood to Spencer Glacier area. If it were me, and I wanted a good bet on a really super sized bull, I'd try the Martin/Bering River area. Cape Suckling is good too. There aren't very many moose over there, but the ones there are huge. Great genetics in that part. Tough hunt though, and expensive. I wouldn't go for the twentymile for a trophy. I might be surprised, but I just don't think there's a bruiser of that caliber in the drainage, and as flat and brushy as it is, it would be hard to find.
Another real good bet is over by Stoney River in the south Alaska Range. Bad thing about all of those are that they all require quite a bit of flight time and that equals a lot of money. There's some pretty good bulls out of Nabesna, too. Check with Ahtna corporation about the land status in the Copper River Basin, though. They have a lot of private lands scattered throughout there. Some good bulls though and if you have a four wheeler, its a ton cheaper.