L78 vs COPO in 69 ('real' engine horsepower ratings)

[black69]

I recently saw a 2016 motortrend L78 evaluation of how much GM was cheating on what the true horsepower is of a stock 69 L78 engine. They found it was under-rated by 50 HP with stock manifolds for a chevelle application. Add headers and you gain 30 HP.

So 375HP was likely more like 425 (which oddly enough actually matches the standard advertised HP on a 65L78 corvette I have, which made me wonder they are both L78s (65 vs 69 so how did they get away with the HP derating for the same engine model?). I even now remember Bill Thomas taking a 65 L78 apart and blueprinting it and it produced more than the GM stated rating of 425. I have the book that showed how he did it somewhere, but I think it got to 450HP, but it was north of the 425 rating obviously.

So now I wonder, and I know the COPO 427 is more, but 'how' much more is it? stock manifolds (from factory) and the headers numbers. My guess it's likely no more than 7.5% better if you look only at the displacement increase and equate that to horsepower gain. But I have no clue really on what the true HP rating are for a COPO 427 vs a L78 in 69, all speculation on my part.

Back in the day, I imagine if you had stock manifolds and the other guy had headers, the race may not go your way looking at the horsepower benefits.