looking at the above list of what belt size to use with the specific pulley, a 660 is smaller than stock, as it should be. the stock length for a gen 3 car is a 663, if you look, you will see. This is the list I have been using:

http://www.grandprixforums.net/threa...elt-size-chart

One thing I have noticed is that if a belt wrap kit is used, a longer belt also needs to be used, but so far I haven't seen any information on what belt size to use with the specific pulley and a belt wrap kit.