[VetteVet]

Your description sure sounds like the pathetic emissions control the 70 and 71 models had that was called the transmission controlled spark. The plug with the two browns and the blue wire were plugged into a vacuum switch on the carburetor that would retard the timing
when engine vacuum was low.
Most of the trucks from that era had these systems disconnected because they did nothing and removing them never hurt anything.

You should have a couple of relays on the firewall or over the fuse box with wires running to them as well.
if you do a search on the forums for Transmission controlled spark, you should get more information. VV