Actually, the alt pulley that comes with most underdrives is SMALLER than a stock 2G pulley. This would spin the alt faster than the stock 2G pulley will. This is to increase alt rpms at idle to maintain sufficient output.
The stock 3G pulley is the LARGEST of the three (UD < stock 2G < stock 3G). Meaning, if you used the stock 3G pulley, you would actually be underdriving the alt when compared with the stock 2G and the UD alt pulley.
Long story short:
The UD alt pulley will not fit without shimming. Some do this without alignment problems. The stock 2G fits and works fine (what I'm using). The stock 3G fits and works fine (what a friend is using).
If you don't have your stock 2G, the stock 3G will work fine. I measured battery voltage on my friend car with all acc running including the taurus fan on high speed. At the idle, battery voltage measure 12.8V. You will be fine with either 2G or 3G pulley. The stock belt length works with both.
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