Even if you don't take into consideration the fan motor in the 20Kw furnace, these two circuits will be drawing 83 amps between themselves while the furnace is running. The math says - leave the furnace out of the subpanel and it will work great.
Move over a couple of other 15A or 20A circuits to give yourself the room in your main panel. I had to go the route of two 200A sub panels feeding off of a 320 amp meter/main panel combination. It cost a little more, but I have all the room in the world for future expansion.