Shaft Length

I bought my hull 2 years back and now have the $$ to put on the Motor, an Evinrude 150. So the question is: What shaft length do I need...20 or 25? Im sure this question seems simple, cuz, well, it is, but i wanna make sure this motor I buy is right. O, plus my transom feels weak, but me and my other friend (combined weight 420+ pounds) hung on it and figured it was most likely strong enough, but we dont wanna drop a motor in the ocean and be stuck in a sinking boat... So I'm lookin for ideas on how to sure that bad boy up. MMMMk thanks.