I always use a battery charger on those cars. They are old and the connectors rusted, the PCM gets very low voltage because of this.
When you flash it, a low voltage can brick the PCM beyonda recovery by software means.
Make sure battery voltage is high, scan it before flashing to make sure the PCM sees at least 13.0 V
It's very very easy to brick the PCM in those cars and very difficult to recover them.