Hay Stink
I think I have followed this thread but maybe I missed something? If your bracket were sealed to the transom than you would already have the max. Flotation from the "box" on your bracket that is filled with air. Putting foam inside would not increase the flotation unless the water seal was compromised.
As you were pointing out about the beam of the 18, it is not just a scaled down V20 like my V17 is, It was designed to be an 18’ boat. I have a friend with a V18 and he rebuilt the transom and installed a 155hp Evinrude outboard on the transom. It will run around 50mph but now he wishes he had installed a 130 or 140 that was lighter due to handling.
As far as the effects of a bracket on a 18’ boat, I think any effects in performance are really more dependent on the boats weight (the more it weighs the more possible improvement) not the length as long as it can handle the weight shift involved. I agree that any info from the seadrive installations in the past would be helpful. They have close to the same setback as a smaller bracket with no flotation. A 1.6 liter seadrive V4 weighed about 385lb, the 1.6 standard 1.6 outboard =310lb. With my 17, I can understand the importance of "More space" the bracket will give you!
My $ .02