Sounds similar to an issue I addressed with my '74. When I first got it, it shifted okay, but made a horrible racket whenever the clutch was engaged. Anyway, what I suspect you're seeing as a "cylindrical tube" is the same as I discovered; the interior section of the input shaft/throwout bearing that directly contacts the shaft itself. It apparently seized on the shaft, snapping the bearing into two pieces. Incredibly, the shaft and everything else was fine. New bearing and it was good to go.
Be very careful when torquing down the input shaft housing when you replace the seals - it is rather brittle, and will crack around the bolt holes if overtightened.
Good luck,
bake315