Converting between Kelvin and Celsius is simple addition; converting between Rankine and Fahrenheit is simple addition. Converting between the two groups requires multiplication, and pre calculator, that’s notably harder.
Also, all your kJ/kg/°C or BTU/lb/°F tables and factors are identical when you swap to referencing absolute zero. If you change to the other unit system, all that goes out the window.
The NASA Vehicle Assembly Building is also a contender.
I’m not sure how many dividing walls there are inside Everett, but the VAB is basically one massive empty skyscraper.