I found some wires on my '72 C10 that look stock, but don't appear in any wiring diagram I've been able to find. Here's the description: They all connect on the inside of the firewall just above the fuse panel to what appears to be a 4 pin relay. One 10G or 12G red that has been cut and is hanging in the wind on both sides. The wires go thru the firewall, two of them go to a small canister on the intake manifold that appears to have something to do with vacuum advance. One small wire (black with white stripe) goes to a connector on the top of my 4-speed transmission, and the other large red one has a stock female connector, but again is hanging in the wind.

I'm swapping the motor with a later model with an HEI, so I'm assuming (but please correct me if I'm wrong) that I don't need the two wires going to the canister.

The other two maybe have to do with backup lights or neutral saftey switch?

From your description it sounds like a TCS (transmission controlled spark) system. What this did was not allow vacuum to the distributor until you shifted into 4th gear. It was an early attempt at NoX control. Later engines just used the EGR valve. You can remove the whole system if needed unless there is some form of smog control in your area for the older trucks