Be cautious of cutting your transom, you'll need to do a neat job and seal it well to prevent water penetration, into the tansom itself but far more importantly, as mentioned by ribraff, you'll then be at risk of swamping the boat when slowing down or with following seas. My first rib was a 5m humber that someone had modified to suit a short shaft engine, the rib was fine and a great laugh with one or two people, but when I put 3 people with dive kit on board as soon as we came off the plane the rib would flood with water almost to the top of the tubes which was dangerous. Even on a choppy day you couldn't sit comfortably and fish because waves would top over the lowered section of the transom.
My current viking tornado is similar. In reverse with the engine trimmed down the water will almost instantly flood over the transom, there is low freeboard where the design of the transom dips to accomodate the engine shaft length, however its better than the humber and actually useful for washing down the deck after fishing or scalloping :-)