Just completed 3.6 swap into 986 and have vibration.

[goldsc_78258]

In my experience that much vibration is probably from a big item eg. clutch plate, clutch or even flywheel look at them first. I would scratch out the drive train since the vibration continues even with the clutch pressed down

hope this helps

[78F350]

What about the engine accessories: PS, AC, Alternator, or even a loose idler pulley? Run it with the front engine cover off at 3k. Watch the belt and observe all those spinning parts.