GM changed things over the years, but for my '75 they used the same harness for both gauges and lights. The difference is in the sending unit and the dash cluster, not the wiring. If you happen to find a tach cluster, that does have a separate wiring plug for the tach.
I am a little confused about your cab swap. Did you swap the full '87 cab with A/C, or did you just pull A/C parts and try to move them to the '86 cab. Do you have the controls and the other pieces under the hood (large A/C plenum, relays, etc.)? If you don't have a complete donor vehicle it might be easier to just buy a Vintage Air system and install it.
I have been working on my A/C on and off over the years to keep it running, so I have a similar interest in understanding all the wiring. If you pulled the A/C controls off the donor truck there should be a wiring pigtail on the back. From there you can just match up wiring colors and run the lines through into the engine compartment. I think there are only 5 wires that go through the firewall for all the HVAC controls.
I could scan in some of the diagrams for my '75 wiring, but it would be better to use one that matches your truck. They changed some of the electric features over the years (3 speed vs 4 speed blower, blower always on vs. having a true off position), and they also changed how the max A/C setting works (in my truck you just move the temp slider far left past a detent, and that is max A/C; in later trucks there is a position on the controls that says Max A/C).
Bruce