Thank you Bob & Randy. I'll use the short one. The short one was the one on the engine, but someone had it apart at some point and the pulley timing hole was 60 degrees off. I'm not sure how they set the timing, but I've corrected the problem. The short one (4.02" overall) measures 3.5" from the front of the pulley to the back of the fan, and the long one is about 3/4" longer, so I think you're both in agreement.
Andy