2000 e430 mercedes serpentine belt won't fit after new tensioner installed

My Belt tensioner recently destroyed itself. It actually came completely apart and the pulley ended up in pulley never never land. I purchased a new belt tensioner on-line and installed it yesterday. I checked the serpentine belt and it was in fine shape so I planned on reusing it. The new tensioner had the pin installed when it arrived so I assumed, install the tensioner, install the belt, release the tensioner pin and we'll be in business. The reality: I installed the new tensioner, and attempted to install the belt but it was too short. I checked and rechecked the routing of the belt and how it was seated on the pulleys and all is fine, but it just doesn't fit. I have just learned there have been modifications to the belt tensioner and the pictures indicate the old tensioner pulley to be smaller than the new replacement. Does this mean a new and different belt? I talked to the local dealership and while they tried to be helpful they were as confused as I am. Any help/advice would be greatly appreciated.