I have an older bike that rides beautifully, and works with a plastic/foam seat with longer rails. However, I just bought a brooks saddle, and unfortunately the rail length is too short.
This prompted me to buy a VO grand cru seatpost with longer setback (probably more than I really need.. 30.2mm possible). I naively bought a caliper without knowing how to measure and thought it'd fit.. but nope, I have a 26.6mm (after figuring out how to read the caliper correctly, I read the diameter as 26.5mm but Sheldon Brown say's an '81 Nishiki International has a 26.6 diameter).
This modern alloy seatpost has a wall thickness of about 2.7mm. Would hiring a machinist with a lathe to shave it down to 26.6, a full 0.6mm, compromise the seatpost? I weigh at or less than 190lbs.