Sorry, not sure why I didnt' get a notify on this, or else I got one and was too busy to respond. Project was on hold while I gutted and renovated my master bathroom, but now that it's 99% done I'm getting wound up on the truck again.
The pinouts I got from the factory service manuals, available right here on nicoclub, although it's not super intuitive WHICH manual you need, the manual itself has a ton of info
https://www.nicoclub.com/FSM/M/2004_M45/
You should be able to find what you need between the M45 and Q45, but probably not the JX45. EC is a good place to start, because it SHOULD be "Engine Control". There's a section that tells you about specific sensors and the pins they use, and a large area for pinouts. The gotcha is, some of the stuff is done in different places and communicates to the ECU via CANBUS. That's the problem I had using the factory gauges (can you imagine how cool it would be to have a functioning M45 cluster in a '55 GMC?) I ended up going to a GPS speedo set from Speedhut instead, although I WAS able to get the trip meter working. (Took it for a ride with a 12v battery a pile of alligator clips and a GPS receiver in another car)
My starting point, just to see, is to find where to apply ground and where to apply 12V, use the factory ECU and transmission computers, and see what happens. It SHOULDN'T know at that point what car it's in, although I expect that I'm going to end up in limp mode when it doesn't detect the passenger side electronic motor mount or the side parking light or something. It'll at least be interesting to play with even if I eventually end up with a Haltech or something. It's a truck, so I'm not intending to race it, mostly use it for parts trips, or local car shows so I can give the SBC people something to complain about.