If the pro's are right! 1.The hydraulic blocks don't rev as high/well or as long. They have proven not as durable, as the 65/66/67-FE 427(these blocks were considered bullitproof) flat tappet side oiler blocks 2. Metallurgy, the newer blocks didn't use higher nickel content material in there block design. 3. the service/hydraulic blocks did not have quite the same construction methods as the 65/66/67 blocks. At least according to my reading. But I'm no expert! But I'm glad I've got a 65
