Over size merc on 3.90 mt tinny

Hi guys
i have a alloly 3.90 mt quintrex dart tinny.
currently i have a 30 hp short shaft 2 cyl,but i want to upgrade to a 40 hp 3 cylinder short shaft with 50 hp carbs on it.
i have a fairly shallow rear transom,my boat is rated to 25 hp only.
so,what mods need to be done to strenthen rear of boat please ?
do i use flat plate or,round tube ??
can you recomend where exactly to make the *** end stronger and capable of holding a jap spec 40 hp short shaft ??
hope to hears some replies soon pls.
thanks
glen

well first off i would post a few pics of the interior of the transom and the hull sides at the rear.
You dont just make the transom board thicker to strengthen up the area.

A touch of welding a couple of brace bars might be in order to take a bigger engine.

40hp is a very big increase over max 25hp !!!

pics of interior will help a great deal


phill