Seattle Jerry,
I'm 6'-8" myself and I fit into the SL barely.
I tired the seat back modification by removing the original seat cover and foam backing. Keeping the spring base and installing a much thinner seat foam and cover from a later model MB. While the mod worked, the cover is not the same as the original. I still have the seat in the attic, and put another original into the car. I've been able to manage with the space and limitations.
You also can try to lower the seat mounts a little in the back, but that will increase the rake(incline) of the seat.
Walter
1967 220SL-diesel