I think it was done that way on Corvettes as a recognition thing. '67 L-88's are obvious by the air cleaner element in the air duct on the hood and the "chicken-wire" insert on the carburetor and the huge road draft tube from the left side valve cover, and have painted valve covers just like the 390, 400, and 435hp 427. The L-89 is simply a 3x2 435hp with aluminum heads, but it was the only '67 Corvette with chrome valve covers, and they left the heads natural to show off the aluminum the owner paid extra for (only 16 were built, which made it the rarest '67 Corvette powertrain).