340 vibration

There is no "balancer" on a 273/318/340 when you think of weights as they are all neutral balanced. The cast 340/360 had weight added on their dampers to offset the lighter weight of the cast crank and the heavier rods. I would suspect the flywheel as that has the most mass, then the clutch if its a balance issue. Have you indexed the crank to the bellhousing? Even the slightest off center will cause the clutch to seek its center every revolution, like a warped driveshaft. 4sp guys could chime in and tell you what the tolerance is. It would manifest itself under "in gear" condition only as a freewheeling clutch would just spin in there.