following. i get an occasional very light chatter on my 78 4speed too, but its very rare & only happens when the stars are aligned properly.
meaning its only at a certain situation like starting from a stop on a hill & not having enough rpm for the short time it takes to release the clutch pedal & get moving. its easily stopped by slipping the pedal for a brief second & raising the rpm a couple hundred. its now just a habit to be sure the rpms are a little higher than just off idle when getting going & rarely if ever occurs. everything in the trans works great otherwise. sorry i dont have any suggestions for fixing it but am curious what others have to say.