"Mine is both freshwater and seawater cooled." Presumably you mean, the engine is fresh water cooled with a heat exchanger which is turn is raw water cooled, but the exhaust is ONLY raw water cooled. (I find "fresh" and "raw" to be confusing, and prefer to say "closed" cooling when there's a heat exchanger onboard.)
Brass pipes? For use with seawater and the bonded galvanic connection to the engine block? I'm not sure that's a good idea. I know once-upon-a-time heavy wall copper pipe (damned heavy wall) was used in some raw water exhausts but hadn't heard of brass being used at all. Have you checked into whether that's going to be a galvanic problem to come?
BTW, NICE drawing. Someone must have access to CAD software.