400's will be too rich. I use 410's in my Pro-X 800 at 4-7K feet and it's still rich. Cooke City is higher in elevation than that too. I think you'll need somewhere around 370 mains with the needle in #2 slot. Yes, you do need the needle in the #2 slot, it will bog BAD in the #3 slot. I have seen this with my own eyes. Who knows, you might even be able to get away with the needle in the #1 slot. Start with #2 first though.
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