I don't know of any aftermarket floorshift kit that could be easily adapted to a Fintail. If you really want a floorshift, your best bet would be to find a salvage donor-car, however the floorshift was a pretty rare option on those cars. In addition, during those years Mercedes used two different manual tranny designs. One had two top-mounted shift levers for the columnshift. The other used a single rear-mounted shift rod for the floorshift.

The columnshifts actually work pretty well if all the bushings are good.
Easest to replace is the ring bushing behind the chrome nut that holds the shift lever. Often bad are the two upper shift-shaft bushings. Those require pulling the steering wheel and upper steering column bearing-plate to replace. More difficult is the shift-shaft bushing in the floor-plate where the steering column/shift-shaft goes through the firewall, though I once fabricated a 'bushing repair kit' out of a bit of plastic antifreeze jug to avoid dissassemblng the column and floor-plate.

A final note - the type of 'baulk' synchromesh used in Mercedes manual gearboxes in those days doesn't encourage really fast shifts. My '60 Fintail 220S shifts easiest with a firm but gentle touch.

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DrDKW