I've done this a total of 1 time with a later model gmt400 and I wish I could remember what was involved. It was relatively easy and I literally do not remember the details.. when I built my '94 3500HD I swapped the Vortec 7.4 complete with wiring harness, pcm, etc. from a '96 donor truck, and I
think the segment of harness for it was standalone. Maybe, and this is a big maybe, it's that additional wiring chunk that sits on top of the block on the driver side of the firewall?
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So you'll need that segment of harness, the cable that attaches to the throttle body, and don't forget to grab those special screws that hold the cruise module to the firewall. They're self-tappers by the way which is nice, and the firewall already has dimples where they go.
I know it's a bit shy on real info, been too many years, but I hope it helps.
Richard