1981 301 to 400 upgrade pt 6

Hello Folks,
           Its been a few months since Ive posted here so here a quick update,
The 1981 SE T/As drivetrain is rebuilt and back in,Engine is a 1978 400 with a non-lockup th350. 1974 heads and Ram Air Spec cast iron mans with a pypes exhaust. Id love to post pics but i haven't figured that out yet.Entire car has been torn down and rebuilt. The body came back last week all painted, Ive installed the drivetrain, rear, all front suspension, the fuel,brake,and vapor return lines all installed, all 4 discs rebuilt and reinstalled, new fuel tank installed all lines run all fluids replaced. Now its on to the wiring, I deal with Factory Fit, They are a great outfit and we need to build an engine harness for this car. Heres the rub I figured I could use a factory harness from a 78-79 400 car. But FF says with the 81 they are going to have to add a tach wire,( ok I saw what they meant when I went out to look at the 301) And That I would have to somehow wire the Wide open Throttle switch to make the A/C compressor work. Now I have dealt whit them enough to take them at there word but just what did the switch do? Any info would help.
Pete  

I think at WOT this was so it could disengage the AC compressor.
Older GM cars had a switch mounted on the carb fixed so that at WOT the throttle lever would depress the button on the switch and that would disengage the AC compressor. What "FF" is talking about is probably for a more modern version of this set-up because '81 is when GM went to full-on computer engine management systems. I think if you are not running this management system on your 400 it may not be necessary to run this wire. Are they making this harness to factory specs for the '81 bird? If so, then I don't think that it's necessary because it's for the computer controlled engine.
Hope this helps.