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70,000 is only slightly large for North Dakota, you will want to size it for the coldest possible weather,
Next size down is 60,000 btu's, that would likely work well enough, cost will be almost identical however. The 70,000 btu furnace will allow air conditioning to be added later if needed.
Tell me the brand an model number and I will look it up for you, a two stage model would be a good choice, that way it will only run on the high stage on the coldest days.