I would like to know this too... I don't have underdrive pullies, but I know my cobra comes stock with a little different belt b/c it has the closest thing to underdrives from the factory. So I would assume that we would need pretty close the same belt...Anyways what belt do you think we need? Completely guessing here but I was thinking something like 89 or 89.5 inch...anyone have any ideas? Another thing I thought that putting underdrive pullies on a car did NOT change the belt length, but taking off the smog pump obviously would. IDK!