I'll answer this the best I can since it's our spec you saw there on our website. My supplier says this spring is 6.5" long. I measured my springs and...lo and behold they measure 6.25" long. So, in the best effort to put accurate specs on our website, that is what I chose to print.
I've already done this exercize but did it again. I looked in three separate Ford books (1933-39 chassis parts manual, 1938-46 book, and the green bible 1928-48). NONE of them list the darn spring length. So, short of perusing thru the Ford blueprints at the archives, I do not have a way to verify what they originally intended.
PS: the springs we sell seem to work fine.