[3macs1]

Here is what I got one time when I asked this question.
Insurance concerns plus General Motors Corporate policies resulted in the use of a lower RPM rating resulting in decrease from 425 HP to 375 HP as a running change for all models except corvette. The L-78 was released for sale in the 1966 SS-396 Chevelle in March 1966 with a rating of 375 HP due to this lower RPM rating and somewhat more restrictive exhaust manifolds, it was otherwise identical to the 425 HP '65 Corvette.( same cam)
The 1965 Z-16 396 Chevelle had the same engine except the Z-16 used a hydraulic camshaft which effectively limited RPM to the lower 375 HP rating.(L-37)
Also all of my 66 Impala/ Biscayne l-72 427's came with the
3880827 and 828 vet exhaust manifolds originally.