For a Baltic/Imperial hops really don’t matter. You shouldn’t get any hops in the aroma, but you want a firm bitterness. I would suggest Magnum at 60 minutes only to your targeted IBU.
If you want to go a different way, you can do something like Saaz, EKG, Fuggle, or even a German Hallertau, but you will likely need more. Magnum or Warrior is my vote, and only one addition.
Answered this on another forum, and I suggested Magnum for bittering too. My own take on the later addition, is to use a British hop with floral/berry/earthy character. I’d recommend Kent Goldings or Brambling Cross. First Gold fits the bill too. I’d stay away from citrusy hops, I don’t think this blends well with the roast.
I’ve recently seen palisade hops described as being like a stronger version of willamette.
I bought a couple of ounces to use for bittering with my back yard goldings as the finishing hops.