You have more than enough pale and probably don't need the aromatic. You can add some more special roast, or some other crystal, just stay away from a lot of dark roasts. I'm putting in .25lbs special b in my english barleywine. You're gonna need a lot more hops than 2oz of challenger if you have 20 pounds of pale! HA! You'll be at the high end of the gravity, and the low end of the IBU's for the style. Granted its a malt forward beer, but that may be a bit cloying. If you have some EKG or Fuggle laying around it, they would make a nice flavor addition around 30min, and aroma addition at the end of the boil.
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