Which route to take driving from Seattle to Boston?

I will be driving from Seattle to Boston in March. There are two of us, so we can share the driving, and we're willing to drive quite late though will stop at night. We don't plan on making lots of stops to sightsee though may stop at a few places for an hour or two if time permits. I have a flight back to the UK I will need to catch from Boston, so cannot afford to arrive in Boston late. We don't have a lot of leeway in when we can leave Seattle and when I have to leave Boston, so I have a couple of questions about planning the trip.

There seem to be multiple possible routes we could take and I have read varying reviews of which is best. The straight shot across I-90 is the most direct route and Google is suggesting 42 hours of driving with no traffic. But I have concerns about what the weather on I-90 might be like even in March. Is the risk of being delayed due to weather in March quite low? I don't want to be stranded in snow in Montana or the midwest, but I'm not sure what it's like in March. Might one of the longer more southerly routes be better in this respect?

Which route should we take, and how many days should we leave at minimum for the journey to ensure we make it to Boston on time?