Any vibration that could be caused by a clutch assembly would occur at a given engine RPM in any gear, not only in top gear at 70 mph. Stated another way, if your engine is turning 2300 rpm @ 70 in sixth gear and you feel a vibration, you should be able to feel the same vibration in fifth gear at 2300 rpm, 4th gear @ 2300 rpm, and so forth. The vibration should be independent of road speed if it is caused by a clutch assembly problem.
If it only occurs at 70 mph disengage the clutch, shift to neutral, and determine if you still have the vibration. If you do it is caused by wheel balance, a bent rim, a driveshaft imbalance, etc.