Personal opinion: The pushrods are not an issue. There are many slants that turn over 6500 RPM. I have turned my 170 to 7200 on occasion. The long stroke is more of a limiting factor. That said, with an overhead cam, you could use a more aggressive lobe profile. With a turbo you don't need a lot of RPM. Use the turbo to make torque. There are several turbo slants that make 500+ HP and don't turn over 5,500 RPM.
