The RAIV cam in a 'cruiser 400' would be a mistake.
Your factory-rated standard 350 HP GTO 400 enginereally didn't make 350 honest horses to begin with. The factory gross HP ratings were a bit optimistic, the output was actually closer to 300-310 HP believe it or not.
400 to 425 HP with stock exhaust manifolds isn't going to happen I'm afraid.
But you can get the engine up to a 'true' 350 HP with a good cam, a re-curved distributor and free-flowing mufflers. Walker Dynomax Super Turbo mufflers (#17749 20" long)are fairly quiet and flow better than noisy Flowmasters.
And drop the compression a bit as
dragncar suggests, trying to run over 10:1 compression with iron heads is difficult and ends up ruining many engines.
Get a custom-built Continental converter, I ordered a 10" unit for my current
400 street engine build-up.
Some interesting threads concerning 400 cams and such by Cliff Ruggles.