It will be difficult without the same tool originally used. Most likely these were ground at the factory by hand using a double wheel grinder that was adjustable for center to center distance between the wheels. They would have used fairly small radius wheels to get up near the spine. See here:
https://youtu.be/BBoI1hmYfbs
Alternatively you can try a setup like this if you only have the usual one wheel sander:
https://youtu.be/apjtYokzgk0