Here's a dyno chart by one of the guys in the FE forum. This is the
closest to stock 428 CJ engine I've seen tested so far. The compression
is down half a point over original, which may be losing 10 or 12
horsepower. The Lunati 10503 is their replacement CJ grind.
You can see where Ford's figure of 440 lbs at 3400 rpm was pretty
much on the money. It has long been said that the 335 hp was
underrated, and here's the skinny. The 335 was quoted at 5600 rpm. We
see here that the hp peak is actually earlier, at around 5100, in the
375 range. After that it drops, and in this case it looks like it might
be at 335 in the 5600-5700 range. So did Ford lie? Not really.
The NHRA factored this engine somewhere between 390 and 400
horsepower for legal competition. I've seen both figures; not sure
which one is accurate. But if you add back the compression loss on this
test engine, you are once again in the ball park.
What's interesting here is that the stock manifolds are not giving
up much of anything against the small-tube Hooker headers. That's a
good testament despite their compromised design.
If you check out the entire thread you will see more dyno info on
more serious engines. See that this forum regular Jay Brown is running
a 1030-horse supercharged FE in his Shelby. Kinda makes all those
huffed smal blocks look a little weak.