Sizing up furnance

My current furnace carrier 80% from 96/97 90000 btu seems to struggle heating up 1500 sqft with 2 story high living room to 73F 4 windows and larger windows on second level. Some nights existing furnace had trouble hitting 70 71 F Also air ducks for living room are located in crawling space that is not heated and has two vents. I was doing some J calculation and my house requires 82000btu for heating. Most my quotes include 90000 btu furnace 80 or 96 efficient 3ton a/c. upgrade from existing 2.5. Upstairs are hotter then downstairs in 99F wether I was getting 80 82 upstairs 76 downstairs. Would be bad to get 110000 btu instead of 90000 btu. 96% efficient furnace ? If I go with 110000 btu 96% two stage variable speed motor would it bad ? First stage outputs 76000 btu second 105000 btu It seems house should get heated to 74 F with first stage and second stage should kick in on coldest days. In Denver metro areas we hit 0F days nights and we broke record hitting -7F. It seems having second stage that can generate more heat would be great. I wan to get 3ton a/c heat pump to heat home on mild days. Sometimes here in coloradowe have 72F day temps in December November. Existing air ducts are 21 x 8 950 CFM in first stage Lennox can be set to generate 900 CFM

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