Hey Sundell, I think that as jman mentioned, it will be a different brew without the Amarillo, Simcoe at 60 and 30 would be fine too. In fact, I have flipped the hops on this and done Simcoe at the end (5 min) and it was great. One of the best things about this recipe is that the grain bill can handle a lot of hops, and tastes good with many different hopping strategies. I don't think 75 IBU is pushing it, but I am a Portland hophead, so my perception may be skewed
I say brew it with Simcoe at 60 and 30. You can dial back the Simcoe at 60 if you want to decrease the bitterness-maybe to .75 oz.
Brew on, Tim