You are correct.
The furnace is most likely operating between 65-70%.
The older furnaces were always oversized. In your case a load calculation should be done, but I do not see you needing anything over 70,000 btu
Currently you just have excessive stack loss.
I cant tell you exactly. The Borg was 1956- 1986.
But the model in the last 20 years of their existence were 4 letters..so I know you are 1966 or older.
Of course this is based on the model being acurate.
That would make sense. I would agree.