first, i think that the 85 300dt has the electronic speedo referred to above.
the 2.47 rear end was installed in a 300td i believe, i am not sure what year. the installation did require some machine work. if i lived at 5700 feet i dont think i would consider the 2.47.

the math looks ok.

a manual conversion to the 5 speed would be nice. i have not heard of anybody putting the 5 speed from the 190s and 300s (E) behind a 617 turbo. i would be concerned of its strength. I am not saying it is too weak just that i dont know an would need more info.

the 5 speed that came in the 123 bodies and some sls is very robust and would be plenty strong, i think.

for a manual conversion you will need pedal assembly, tranny, shifter, flywheel, cutch assembly, and the driveshaft would need to be adjusted for length (probably). as a trade off you could avoid changing the rear end. the 5th gear ratio in the older style 5 speed is .813 so take that times your rpm at a given speed to get the resulting rpm if a five speed were used with the same rear end.

for me a five manual is the ultimate, unless i could figure a way to jump right to a six speed. i noted on the performance section some guys contemplating a six speed conversion kit but the cost was in the 7-10,000 range i think, so this would be a very long payback.

the older 123 five speeds go for i believe 2-4,000 used... and are rare as hens teeth, perhaps rarer.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.