The speed limit is now 75 in ND/MT/ID and 70 in Washington. I think 65 in Minnesota but I'm not positive.
I'd only spend one night, crashing in Billings. That's two ~11 hour days, so its a haul, but there really isn't much to see along that trek. The prettiest part of that, western Montana and Idaho, you should hit during the day if you spend the night in Billings so you'll be able to see it during daylight hours.