If you have a new transome, now os the time to make a change, if youare going to. A #3 will not bolt up to a trs gimble and props are a lot more expensive.
A Bmax/Imco, or any Bravo type drive, you do away with the tranmission. Which will change the characteristics of the boat. Maybe a good thing, maybe not. It will change because you have to move the engines back, or install driveshafts and a second engine mount.
Another way to go would be the Konrad drive. The Konrad drive will bolt up to your TRS gimble and use your current trans.
Really, I think you would be better off with 600hp going thru a TRS than a Bmax.