You said your total heatloss for the house was 76,329BTUs.
A 60,000BTU input 90% furnace has an output of 54,000 BTUs.
Thats 70% of your total heatloss.
Without a room by room, or floor by floor load calc, I can't tell if your first floor needs 42,000BTUs, or 52,000BTUs.
But, insulated the way you say the new 1140 sq ft of first floor is.
That area probaby doesn't need more then 26,000BTUs. How much the rest of the first floor needs is hard to say.
Will it need more then the remaining 28,000BTUs that a 60,000BTU 90% has available(31,000 if 95%)..
A full room by room load calc will tell you.
While it may seem like it takes a lot of time to do one.
Keep in mind. That a correctly sized furnace, needs smaller duct then an oversized one. So it also saves you money on the installation cost.
Making the time spent on the room by room load calc well worth the time.