The more HP parts you add to an engine, the more critical good flowing exhaust is. Most published exhaust tests are done on stock engines and not on modified engines. The more an engine is modified, the greater the gains with good exhaust. This even more true with N/A engines that have bigger cams.

We had a guy with a 383 similar to mine. 234/240 hyd roller, AFR heads etc... Very similar exhaust setup, aluminum manifolds and short stainless risers with welded extensions into the tip. Poor man's dry exhaust esentially. With 1.75" dyno headers it made 491hp. With the boat exhaust 480hp. 11hp @ peak is all gained with dry auto headers vs dry boat manifolds. Other tests have showed on a relatively stock Merc 350, cam, carb, intake upgrades that a stock GLM manifold/riser setup was only down 3hp vs Revolution Marine/EMI setup. BBC might be a different story but at 450-550hp you can easily make it with exhaust extensions.

"Had a guy" - Ryan, I'm still here and ticking. Keep in mind it was not a stock manifold vs Kodiak test. And I am running Trickflow G1 heads (wishing they were AFRs).

Even stock riser's (merc) are on the borderline of reversion. So,
any improvement (mods) is a plus until you get to the 500 hp +.

Meant to say Had a guy from SW forum. But its kinda a no no around here to mention it.