Spokane to Calgary with heavy load - best way?

You are overloaded and underpowered in the mountains by your own admission. I would not do it but it's your call. If you really need to attempt this journey 2 then 95 and 3 would be the most direct route. I-15 and I-90 are no easier with your set up, I drove it with an underpowered fuel truck. It was not fun.