At my old job my boss built a 440 for a guy. It had Edelbrock heads and a cam with 238 @ .050. On the dyno it made 538 horse and 540 ft lbs. with headers.
With HP manifolds it made 495 horse and around 500 ft lbs. On a milder engine the difference would be slightly less.
I had a 69 Valiant that the HPs wouldn't fit. The driver side hit the firewall, too much to hammer it out. The car had the Shumacher mounts, and they should have fit, but didn't.