I promise this is my last question....altitudes along I-90

Penny, you're not being a pain. :) Haven't done the entire route on 90, but at least from Missoula east, I don't recall any altitudes or grades on 90 worth worrying about - even 80 is higher, and 70 is the one with all the really high altitudes and long climbs. I think you'll be fine. Probably the biggest climb you'll have on 90 will be just east of the Seattle area.