GTP will be fine..... I like to use 3rd gear when I see the boost gauge registering, but if you're doing 70MPH you should prolly keep it in D. If you find it shifting again and again, just put it in 3 until you're over the grade.
Where you going in Oregon? I'm in Portland, and we may have a club event happening if and when you're in the area. If not, I would enjoy having a soda, or whatever. Let me know.