Maybe the HEI Distributor Pickup Coil can supply two ignition modules, one output to the 4 pin ignition module in the HEI distributor and one output to an externally mounted 7 Pin EFI ignition module. Perhaps the 7 Pin EFI Ignition Module will provide a 0 volt ~ 5 volt square wave RPM output signal?

Hei7pinDiagram.gif