Second, unless your home has no thermal mass and tons of air leakage you don't even have to worry about design temp - it happens for such short periods. (*** Make sure you don't go for crazy setbacks when it gets really cold - your home may not move UP from 50, but it'll likely stay at 70)
If it goes 20 degrees below design temp it'll take a LONG time for your furnace to NOT KEEP UP. In other words, the heat already built up in your house will be lost slowly over that period and MAYBE your 70 degree house will drop to 65 - after a few days. In that once in 50 years situation, if you want to keep 70 degrees - buy a couple space heaters.
You want as low a second stage as possible - that's why high end furnaces now have a 3rd stage. MOST OF THE TIME 3rd stage is oversized! You want to get as close to properly sized for "most of the time" on your low stage.
Don't spend more on equipment and more on energy for the next 20 years for a once in 50 year event that lasts a few days! Put a sweater on for heaven's sake! Get a blower door test to know your infiltration. Fix it if it's bad.