My originals weren't rusty like the ones shown. Mine were bent and warn about 1/2 way through the bolt. Replacing them made a big difference in the ride quality.
Outside to inside measurement is a fairly common way to measure between centers. Much easier than trying to figure out where the center of a hole is. Just remember to measure to the same point on two holes I.E. Outside to inside or inside to outside. Or inside to inside + 1 hole diameter. I've used all of these methods from time to time, even when I part of a land surveying crew when centers were hard to find, manholes for instance.