I don't have a serpentine belt, but this is what I think - you'd need a shorter belt
Long answer is: Swapping your stock pulleys for smaller ones will add more slack than the tensioner pulley is able to do it's job with. Now, how much shorter would the belt need to be? Not a clue... ask the company you're planning on buying the pulleys from
To answer your original question - yes you do need a different length belt.
However, I would strongly recommend NOT putting these on your truck. I installed them twak ) and they caused way more trouble than they are worth!!
They would work great if you are only driving a 1/4 mile at a time, but for daily use on a truck, they suck!
If you are determined in using them, I will gladly send you the HyperTech Power Pullies that were installed on my Bronco.