I have been (modifying)extreme down river racing kayak hulls into a british seagull outboard powered racing displacement catamaran. The only wizdom that I can offer is that if you think about it, an outboard motor is essentially one big lever. The propellor (push) is at the end of a long shaft and that equalls major torque.

You can get away with alot if you use a really small motor and throttle up slowly, but I and other people that I have spoken to have experienced much snapping of beams/transoms ...

However, don't despair. Recognising that there is going to be a torque problem is the answer. Think about it and resolve it. I successfully (and simply)resolved the problem by bolting a little piece of angle aluminium from the gunwhale (sheer)to the top of the beam where I mount my motor. Now I can get away with blue murder. Gimme more horse power!!

Good luck

Pete