How did GM go from the 425HP L78 in 65 to the 375HP in 66. Did they change the cam?
It was all corporate politics as the 1966 L72 was originally rated at 450 H.P.
An AMA agreement stipulated that max. H.P. for passenger cars would be capped at 425 H.P.
So it's no coincidence that the 426 Hemi, 427 Ford Side Oiler and the Chevy L 72 were all rated at the exact same 425 H.P.
The L 72 H.P. rating was reduced to 425 and the L 78 reduced to 375, all on paper or air cleaner decals..
The 1965 Z16 Malibu, 375 H.P., 396, PRO L 37, was used only in the Z 16.
The main difference was it's use of a hydraulic cam, where the L 72 and L 78 were all solid lifter.
As Corvettes were sports cars, not regular passenger cars, they were allowed to exceed the 425 H.P. limit.
Obviously, the 1970 only LS6 Chevelle ignored the AMA agreement as it was the end on the era.
I refer to H.P. ratings of the day as a means of Engine identification, not a true representative of actual H.P.
Several Engines with lower ratings are more powerful than similar Engines with higher ratings.