Peter,
I am not au fait with the 400 engine but may I suggest you contact Spencer Lane Jones who do offer very impressive tubular exhaust systems for the V8s. See
https://www.spencer-lj.com/services/...evelopment.htm. They’re not cheap. My experience (on a 410, but I understand they also cover the 411) is that one significantly improved the breathing and responsiveness on a car that was already well up to the mark. I speak only as a satisfied customer.
The other advantage that I perceived over the ghastly original cast manifolds is that the exhaust heat is no longer concentrated right up under the heads and the engine bay runs cooler. This might be further enhanced by wrapping insulation around the individual pipes - there is a win-win here as there is a further potential gain in performance to be had if the engine can ingest reasonably cool air, which is pretty uncommon stuff under most V8 Bristol bonnets!