RE: fpe 5.8 with the shakes...
I have a low run time Sachs 5.8. It's one with the good crank. I maidened the plane that it was on this weekend. Honestly it doesn't shake that bad. Nothing like you're describing. I would say where the vibration is the worst is at lower idle. As the motor peaks out it gets very smooth. I don't think anything of it, I've seen BME twins shake just as much at idle.