As Dennis mentioned, the headpipes are the most important part of the system when running iron manifolds. That first 90 degree bend coming off the manifolds is critical. Dennis has done some dyno testing in this area so he could give you exact numbers, but an educated guess would be 7-10 hp. Having a mandrel bend here is beneficial.
The further back you get in the system, the less critical it becomes. The 2" inlet mufflers are likely a bottle neck, and at some point off the manifolds you'll be reducing the size of the pipes, it will choke some HP even on a mild 400 build such as yours. Running smaller tail pipes after the mufflers aren't as much concern.
It's up to you and what satisfies you. Personally I've learned that even on a near stockish build such as my example above, exhaust is a big player and I don't like giving up anything, even on a street car that never sees the track. You're 400 probably isn't too far off from my own PS 400 build I mentioned above.
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