The transom of the 30 inch bracket is taller then the transom of the 25 inch single engine bracket. If you were to mount a 25 inch motor in the center of a 30 inch bracket the anticavitation plate of the 25 inch motor would be obstructed by the transom of the 30 inch bracket. All flotation brackets are mounted as low as possible on the transom of the boat to gain maximum flotation when at rest. There are other variables as well for instance the deadrise of your bracket will influence the flotation chamber dimensions which will influence the mounting height of the bracket....for instance a bracket with a low deadrise and a wide flotation chamber may have to be mounted higher on a transom (as you eluded) where as a bracket with a steep deadrise and a narrower flotation chamber can be mounted lower on a transom. It all depends on the bracket dimensions and the boats transom deadrise. Most boats have a drain plug coming out the bottom of the transom which means you will have to mount the bracket just above this. Confused?
strick