2000 6.0l swap into 1990 GMC 2500HD using the stock th400.

[Lurch228]

The 3L80HD (aka TH400) in your '90 should work using the PCM Kick Down functions, but not directly.
Technically it would work with a relay wired between the PCM to the detent solenoid. If you wire it direct the current will be too high and will most likely smoke something in the pcm. The next challenge is correctly setting up the programming (.bin) for a non-electronic automatic but still have the kick down function.
I am sure there is a P01 .bin and .xdf out in the wild that will allow you to do what you need.

On another note, if your 6.0 is a LQ4, the 3L80HD should bolt up without any of the crankshaft spacers to keep the torque converter spacing correct.

As per my post I am trying to use the 2000 pcm to control the 90 factory installed relay for the kick down. As the 90 pcm controlled the kick down using the TPS !
2000 pcm has what appears to be 2 wires for downshift, and based on the 1344 OS which has 3 maps based on speed and tps for 4th to 3rd, 3rd to 2nd , and 2nd to 1st down shift. So tying the 2 2000 pcm wires each through a diode to the negative of the factory installed 90 kick down relay!

Also 1999-2000 6.0l ls was only LQ4 cast iron head, aluminum head started in 2001!

What I'm trying to figure out is are the transmission shift solenoids A & B used for up shift, downshift or both.
Does the 3-2 shift solenoid control(which is not present the on this vehicle) also control all down shifts in any gear. If I switch to a different OS would this function work if I add the wire to the pcm since it is not there ( pin is empty at pcm)?
Haven't been able to find anydefinitive answer to this!