All MB trannies of that era are one to one final drive. No overdrives so the tranny won't help.
The differential ratio is recorded on the rear face of the housing, visible even with the cover in place (no disassembly needed). View from the rear in the lower right corner, I think.
I'm guessing but I think the 300D 115.114 would have the highest ratio in that body.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician