Hallertau Tradition and Challenger are both spicier than Willamette. Won't matter much on using 0.75 ounce of Hallertau for the bittering, but I'd back off the Challenger to 1/2 ounce. In fact, you could use 1/2 ounce of Hallertau @ 10 instead. You won't have the earthy notes Willamette gives, but the brew should be good.
Willamette's AA% varies more than most hops, it can run from 3-6%.
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