Inboard exhaust pipe

[dakota]

  does anyone know about inboard exhaust "flapper valves"? I have a 1974 Aquasport Inboard 222. I an rebuilding this boat(new floor). the engine ia 5.8lt.(351.cu.Ford).well the exhaust pipes(bronze) had little hard rubber(pinned) flapper.they need replacing but I cannot find a replacement to fit. orignals were just like they designed for .backflow of water into exhaust pipes . now the pipe measure 4 1/2in. od. inside od.is 3in. only thing in my area is a rubber clamp on style.it slip over the outside of the pipe(4.5in.needed) w/hose clamp. only I know the mfg.makes 3",4" then 5-6". so NO way to work correctly! found a marina haveing a close item.my boat's exhaust casting has a flat on top of i.d (3") with 2 small screw holes (2-56,4-40?). so much for discription, any help out there will be welcomed!! can't go without these "flppers" working properly!! know this boat has become not a real pain in the ass! found 2 Asports  19'6s w/trailer&engines(outboards) for 3Gs or less.!! Help please.  dakota