Contact D and D Marine (I am in no way affiliated with them). I just got an oversized tub bracket from them for a transom job I am doing on a Robalo 2160. They are great to work with and will make what you want; not sell you what they make. This bracket has a 48 inch wide tub, 6 ft swim platform, 30 inch setback, 2 Armstrong inspection ports, and installation kit consisting of all bolts and two 6 ft lengths of 4 inch aluminum channel. Total price was $1700, plus $167 shipping.
This would make that boat a whole lot better in the stern. I have calculated the 48 inch wide tub provides about 525 lbs of floatation, which should be more than enough to offset your current motor. This would be an easy way to fix the problem, no invasive cutting on the transom, and only a couple days of work. By the way, this is their twin engine bracket drilled for a single engine installation.
Let me know if you need any more info.
Dan