Its especially strange that kickdown doesn't change the shift point much. I would verify that power is available at the kickdown switch (terminals 1 and 2 should be hot without closing the switch). Power comes from the A/C compressor control module (strange as that might seem).
If you get internal, you should verify that the control pressure piston goes full travel when someone floors the throttle. If it does reach full throttle, loosen and retorque the valve body and as Mike said, adjusting the allen screw will change the shift point, about 5mph per turn.
I have seen a couple 300SDLs that exhibited absolutely low shifting. Such that no external adjustments mattered. I fixed both with a valve body. MB provides rebuilt valve bodies so cheap, for most units, that very few techs have ever learned enough to fix them properly. One of those people visited this site for a time from Australia (where they can't buy rebuilt VBs). Another is Mario Arisides in Miami. I wouldn't be surprised if that link above gets you very close to Mario. He has consulted on most ATSG MB projects, (I hear). His business is called Independent Transmission.
Bosch Master, ASE Master, L1
33 years MB technician