I was leery of that as I worried the vibrations from the engine could stress the manifold connection seal or other joints. But I can't see an easy way to connect a brace to the engine that would be effective.
I won't have too long a riser but the weight of the parts are substantial. Yours and Todd's are having no problems. Maybe I'm overthinking it because my manifold plate is about 3" wide and 4+" long with 4 bolts. But the bolts are 5/16" which seems a little small.
I take it the top of your riser is well above the waterline? Do you have a vented loop on the water intake line? If so, where?
Your hull is similar to mine. What general shape does your hose to the stern outlet take? Did you get a high loop before the outlet?