i renewed the bushes with cutting the top of the gearbox cover. Afterwords i repaired it with flat strips aluminium bolted in the 2 sides along the cutted line. Put some tape over it for fumes and the carpet… all perfect again ! it isn't a structural piece ... just tin plate .
I was indeed pushing the hole gear shift support against the rotating axle flange.... with the noise of a near exploding gear box ... rubbers were complete worn.
If needed , i can easely acces them again.... just some screws