Markus,
I have a similar problem with my 1948 Chrysler with the M-6 and Fluid Drive. I don't know the solution, so I will follow this thread and see if I can also benefit. I've never been able to get the kickdown to work and if I start out in low range, it will "back out" or grind after it upshifts. Once I shift to high range, it works fine. I also read the Service Reference Book on the Imperial Club site and can't say it helped me to understand any better. I'd recommend reading the book "Transmission, brakes, shock absorber tips" Volume 4 number 12 in the 1951 series on the Imperial site as it comes at the transmission from a different angle. I hope we can both benefit from the wealth of knowledge here, as I am also facing the task of putting a Fluid Torque transmission to work in my '52 New Yorker. I've driven a friends' '51 New Yorker and it was a very fun drive.