Years ago, I took some 5" diesel steel pipe and made a double 90 degree ell that I was needing in my boat. Then I got it hot dipped galvanized. It never failed. Also, those cast iron risers cost me a few motors when the gasket failed. Just my 2 cents.

how did they cost you a few motors? what did you do to change it?

You need to change teh gaskets between the manifold and riser at least every year or 2. If they fail, water will seep back into the engine. It will hydrolock and bend the rods.

The exhaust will take the path of least resistance---which is straight out the tips. Very little will go out the prop even if its mixed.

BTW, a stock 454 in a boat will be quite a bit quieter than open headers in a car. The boat exhaust is muffled with the water mixing with the exhaust.

The cast iron risers to manifold connection will get eat up with rust on the i.d. (I was in salt water use). Water can leak in to your engine and you can't see it. Merc. tried to resolve this by making a SS riser. However, the SS riser expanded differently than the cast iron and again the gaskets failed. My final resolution was to replace the cast iron manifolds with Stainless Marine units. No more issues.