Sounds like a good project. The 680 hosels are .355 but installing the C-tapers you have can be done easily. I recommend having a specific .370 reamer drill bit for the job but a new sharp 3/8 bit will do even though it is .375. The good thing about the 680s is they are very soft steel and the existing bore acts as a guide to make a straight drilling easier. After that, it's just a normal shaft installation process. I will usually check loft and lie specs afterwards as a finishing touch.