Thank God.
I thought this thread was going to be about using a 4-pin HEI in a vehicle that came with a 7-pin module. There's a thread on another forum where some guy is running both modules, so the computer thinks the ignition is being controlled electronically--and therefore doesn't set codes--when in fact he's got the centrifugal and vacuum advance working, in the old-style coil-in-cap HEI that's actually in the engine.
The only thing I can add that's on-topic is that I've got a similar or the same trailer connector on my '88 K1500; but not all the wires are connected. Never spent the time to figure out how to add the extra three wires beyond what the harness-adapter kit provides at the connector in the LR corner of the vehicle.