Lets break this apart to narrow the possible causes.
driving my 70 Duster the other day cruising in 4th gear and the car lost all power.
Driving during the day, then lights were not on. Was the heater fan or wipers on? Radio?
In other words, how do you know for sure it "lost all power"?
I turned on the lights and they now came on.
Did you try turning on the lights, before opening the hood?
Got in and turned the key and she fired right up. I was able to drive home without any further issues. Would a temporary loss of ground cause all electrical in the car to shut down.?
Possibly. The alternator case is grounded so the engine could continue running without the battery ground.
Did you notice what did the ammeter indicated when the car died? How 'bout whern you restarted?
Things that could kill the engine while driving include:
vapor lock (we're still on winter fuel and the weather hasbeen warm)
Loose connection on the coil
Loose internal connection on coil - usually fatal but sometimes intermittant.
Any ignition wire grounding out
A loose connection on any ignition wire or feed.
Losing all power requires losing both the battery and the alternator power feed.
