why does my engine die when i unhook the battery cable?

Jim, BTW, DUHHHH!!!!! I'M not a child nor completely incompetent! It was by accident that the cable came off. This is an old farm vehicle that hasn't been in my care all it's life. And I'm WELL aware of the "advancements" we've made in automobiles, sir. Thank you. Dannyl, thank you. The only modification was the distributor. The engine is a 351m. I removed the ignition box and external coil and used the power wire from the old coil to power up the new one. Maybe my alternator was bad out of the box.

Connections and wire deletions would be a technician's focus. Since you do not have your battery connections secure, and you are creating arc as well as excessive resistance, and your duraspark wiring had to be deleted as well as your coil connector, not only would you need to be certain that your deleted circuits did not have other components in play, but your wiring for duraspark can have useage at the starter relay and the ignition switch via the same harness. The resistor wire for the ignition coil was likely taking on engine heat from lack of shielding. a common cause of low spark in older duraspark equipped vehicles. Some had alternator coil and starter relay connections. That's why noone can say for sure without the EVTM and the vehicle to inspect and trace the wiring. (Electrical Vacuum Troubleshooting Manual, Helm publications) Some libraries, and small town dealers still have this info available. Automotive databases attempt to have it but it is difficult............ This will be the area your tech will inspect and discuss first....Connections, their resistance, volt drop, and alternator/reg. testing.

That is , in my humble opinion.