Since the 123 5 speeds are hard to come by... I wondered if anyone has thought about adapting a Borg warner overdrive to their benz?

I used to have 60 willys Jeep wagon with a Borg Warner ( studebaker) overdrive adapted to the transfer case. ( see pic)
I was at a car show lst year and saw the same overdrive adapted to the rear differential making it a clean easy install rather than at the rear of the trans.
I'm wondering if there is enough room for an overdrive mated to the rear output flange?

Anyone think I'm nuts?

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81' 240d 4 spd 140k on the clock