Take a look at the wire going to the HEI BAT terminal. It should not have a woven or cloth-like coving on it. If it does, it's the old resistor wire and needs to be replaced. A resistor wire will measure 12 volts if removed from the BAT terminal and confuse things. The HEI does not like a resistor wire.
If it does look like it has been changed to a normal plastic wire, how was it attached? Soldering it on at the bulkhead connector can cause problems. Easiest way to hook up the HEI is to run a #10 from the IGN on the fuse block.
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