Eh, I'd agree with what they said. A 331 with heads that flow 300+ CFM intake, 220+ exhaust, a properly matched cam with no less than .550 lift, a decent size intake like a Eddie RPM 2, SysteMax, or TrickFlow R, 10:1+ CR, and with all the supporting goodies should make peak HP around 63-6500 RPM and achieve 450 in theory.
MM&FF just made almost 460 hp with a carbed, 10:1 347 with TrickFlow 185 CNC heads and an OTS cam. That thing made peak HP around 6 grand, IIRC, so there was more to be had with a bigger cam and higher RPM. It's not rocket science, you just need to let the thing breathe and match the parts right.
What you also have to realize is that an engine on an engine dyno has NO accessories, NO water pump drag (usually), HUGE straight-through dyno exhaust, NO heat soak, and a constant supply of fresh, cool air. Under these conditions, 450 hp on an engine dyno might equate to something like 375-390 rwhp. If you've ever seen a "stock" 5.0 run on an engine dyno, they usually end up making ~250 hp, even though Ford tells us they only made 225 in factory trim. That 11% difference comes from lack of accessories, free flowing dyno exhaust, etc.