.....just wondering if anyone has ever checked these measurements against their cars?
I'm not picking on you but at 57 units per hour no one measured anything in production. A friend worked in a dealership bodyshop back in the day and he laughs at stuff like this. Crooked, off-center, overspray etc. One of the more obvious errors made on the line was to use the vinyl roof position for the rear stripes on a car without a vinyl roof. A few survivors have this error.
Common striping errors in restoration are '67-'68 rear stripe layout on '69s; extending the stripes onto the trunk lid edge on a car built with spoilers; using the wrong color for a particular trim combo.
The spoiler was installed when the stripes were applied. Here is an excellent photo of how the factory did it.