If that 280 magnum cammed made a real 400hp, I'd be surprised. Dyno's can spit out all sorts of numbers, numbers sell parts and magazines. None or few of those engines usually ever end up in a car to prove out the claims.
The bigger MM lobed one would make in the 400 range. That's a pretty aggressive lobe family and not really easy on valvetrain from what I recall.
I've been involved and built a few of those xe268 cammed mills that the mag build started with. NONE ever made 390+hp... more like 335-350 depending if a 340 or 360 inch engine. Proved to be the power HP point when run at the track for MPH/wt.
