You are in the ballpark. Heavyweight passenger cars ranged from 40' to about 81' over the buffers - confusing, sometimes, because dimensions are often given of the interiors.
Harriman cars included a 40' RPO, 60' coaches, 72' diners - you get the idea. Usually "all baggage" cars were shorter than coaches or Pullmans.
Yes, scale passenger cars ranged over quite a wide array of lengths and were not the specifics that seem to have grown into some metric by modelers.