Your sib should use a short shaft as that is what it is designed for, at slow speed it should be ok but don't forget that the leg is longer in shallow water.
If you want to use your boat on the plane obviously you will be putting more strain on the transome and also the trim will be effected so I guess you will need to trim all the way in, but it still may ride high at the bow.
If it's only going to be used on a sib you should really get a short shaft, AJ is only trying a long shaft as he also uses his as an auxiliary engine for his rib.
One more thing, you need to check with your insurance company that everything will be ok and you are covered.
Hope that helps....