This question has hung in my mind for some time now. It seems that there are cars that fit the description, but are longer than 72' but shorter than 84'6".
I finally found the reference. It is in Patrick Dorin's "Everywhere West, the Burlington Route".
On page 76 is a table of cars used in Zephyr trains. There were a series of coaches used in pool service that measured 80'2" over the buffers. These cars were stainless steel, corrugated Budd cars. These cars were not the short, articulated cars used in the early Zephyrs. They were built in 1940.
Two of these cars, "Silver Alchemy" and "Silver Castle" were rebuilt in the Burlington's shops into dome coaches. "Silver Alchemy" was appropriately transformed into "Silver Dome" in 1945. It was the first modern dome car. "Silver Castle" kept its name, and was converted in 1949. The cars kept their original length after conversion, 80'2".
"Silver Dome" was produced before the postwar flood of passenger cars arrived from the big carbuilders. These first two dome cars differed from later Budd products by having angled, flat glass window panels and longitudinal seating (facing the windows) under the dome. The flat glass domes bore a strong resemblance to those produced by Pullman-Standard.
I was fortunate enough to have ridden both these cars in "Twin Zephyr" service. In my opinion, the quality of materials and workmanship was every bit as good as the later factory-built cars.
So don't despair. There is a prototype for your "shorty" streamlined passenger cars, even shorty dome cars. I run some of these on my N-scale model railroad, and, to my eyes at least, they look just fine even mixed with standard length cars.
Since these "shorty" cars were produced by Budd, there is a good possibility that similar-sized cars were used by other railroads employing Budd equipment. You are on your own there.
Les