Underdrive pulley kit comparison

Yes, there are differences in size. Some pulley diameters are designed to slow everything down; some are designed so the crank pulley slows everything down, but the water pump and alternator pulleys slightly speed those components back up to help with potential charging/cooling problems; some speed the alternator back up, but not the water pump -- it's really all over the board. If all you're trying to determine is proper belt size, simply go ahead and install your delete kit/remove smog pump, and then route a piece of string into the belt location while a friend uses a wrench to hold the tensioner in the middle of it's range. With that length as a guide, go to your parts store and buy belts in the length you measured, and 1/2" longer and 1/2" shorter. One of those should fit properly - return the other 2.