4 speed to 4 speed od clutch chatter

The 3.09 low sends 16% more torque into the rear springs. I bet you a nickle your rear springs are bouncing. Put some clamps on them and try it again. Or put a big old wooden block between the snubber and the floor, for testing purposes.

But that is just a band-aid.
Your clutch does have issues, the 2.66 just masked it.
Your finger heights are very likely not the same.