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Ability to work on the car is why I suggested the 2 post. Everything is accessible, and you can still park a car under it. Having 3 screw stands will allow you to pick up any part of the suspension if ever needed.
If you look at all the builders a 2 post is what they work with. Just go through the builder's sites, and see. good luck
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20mph is not fast, unless you are doing it in a 3/2, 1000sq. ft. house on 10 ft. waves! 
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