I understand what you are saying but the transom is not something that flexes so much like a ski. How much could a transom move or flex when you have everything bolted in an connected.

actually they flex more with foam core . . . much more especially in a solid sheet. I'm not trying to freak you out but rather educate yourself and others based on my experience with many many transoms and a few race boats that failed. When i buy full sheets of foam and full sheets of marine ply and lay them across saw horses the foam sheets droop way more, dont think for a second those properties dont translate. Again i am sure you will be fine, my only point to the general reader here is that when building a base for a high load high HP high torque transom foam is not my choice and likely never will be. 99% of the time in the real world composite is great but my stuff occasionally is required to perform beyond that and i am 100% sure mine in marine ply will not fail.