Adding a second alternator is not always the best idea. It will causes an added load on the engine, not to mention a mess of brackets and possibly belts. Depending on your needs, an upgraded alternator should do the trick. Check with High Output Alternators (link is below). I think they have a 220 Amp alternator that fits. If you need more power than that, they might be able to custom make an alternator for you.
Tip:
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