If you want and if you are feeling adventurous. You can buy the under drive pully and leave the alternator pully alone or even increase the size of it. Then take a piece of rope and route it just like you would the belt (of course put tension on the idler) Mark the string, take it out and then measure it. That would give you the length of the belt. I would just leave it as is in the kit. They have designed it so that it will still have sufficient amperage to charge everything. High amp alternators will also help. As far as your plans, it wouldn't be a problem.