[pizda]

Some better parts would actually be keeping what you already have.... the stock pulleys. Running the 100a alt with the underdrive on it would be like running the stock alt with stock pulleys. The underdrive will just slow the alt down, and in heavy traffic / low engine speeds charging may become a problem. I would just bead blast the stockers if they are in rough shape and have them painted or powder coated.
Sometimes the stock stuff actually works better then you think!