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If still stock, it should have EEC-IV. With EEC-IV, the distributor should have an ignition module at the base with a few wires coming out of it and no vacuum advance. Along with this, you'd have an electronic feedback carburetor.
If it was the non-computer controlled Duraspark system, it would have vacuum advance with the ignition module on the inner fender.
If it is the original engine and your sure no one has changed the ignition system. Look on the emission control/vacuum diagram in the middle of the radiator support brace and it should have ECC-IV printed somewhere on it. If my memory is correct it is on the right lower side of the diagram as you face the front of the truck.
This leads me to another question - If I were to install one of the Offy 4 bbl manifolds and 4 bbl carb, and I do have the EEC-IV system, then would I need to change to the Duraspark Ignition?(or some other non-computer controlled ignition)