In my area, most 1600 sq ft house of the same time period could be easily heated with a 60,000, 90%er. We usually get a little colder.(but not these last 2 years)
What size was the old furnace. A load calc can be fudged to say a size a contractor wants it to say.
Is it wired for 2 stage operation, or do they have it on a timer that it goes to second stage every cycle.