There was a fantastic 383ci engine article in an old Mopar book, ''Musclecar and Hi-Po engines''
Chrysler themselves got Victor Edelbrock to do a 'Hotrod' article on a NEW factory 383ci engine in 1967!
They had been hearing of ‘bad press’ for the 335hp ‘383ci’ so 'MOTHER MOPAR' wanted to put the record straight lol.
On the Dyno, a brand new low mileage 383ci made only 280 horses...
So with lots of machine work and the right camshaft, they got it up to 420 horses.
The article does not say which heads were used 516 or 915 (440) heads?
The biggest problem being was 'factory over-tight' clearances.
''Freeing the horsepower'' was just more piston to wall and bearing gaps, worth 100hp...