You can keep the stock tensioner. As for the belt length that is going to vary on every setup. I would sugest just getting a piece of string, keep it as straight as you can, mark the length and measure it then go to a parts store and ask for a xx" belt and how many ribs you need, they should be able to get you one based on the part number. If you want you can buy a size up and down from what you measured as it will not be exact. It would be easier to visit local parts store and have an extra car because you are going to need to make a few trips regardless.