The MX5500/MX6000 series uses a single (presumably digital) pin to transmit and receive stand data, which connects to the AV link bus inside te television set. (Service manual, p. 17).
The MX8000 uses an analog system on the 5-pin DIN connector, with a feedback potentiometer to determine the stand position and two active low outputs to control the stand (MX8000 service manual, p.10), so the two wiring schemes are incompatible.
You might be able to combine a stand control PCB from a dead -5000 or -6000 series PCB with the existing stand motor in the 8000 stand, since they run off the same 14V supply, that could work.