Ducts may be undersized but as long as it gets proper airflow in heating mode, should be okay as long as you can tolerate the extra noise in heating mode.
The 5 ton drive (maxes at 1900 cfm at 0.5") should be able to move enough air for heat on medium low or medium, but static will be through the roof.
Minimum heating cfm is 1050 for 65F temp rise but really it should be 1200+ to not stress the heat exchanger.
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.