Engine is starting, but 12volt power to TFI/Coil is being cut when key is in RUN position, for sure, not a guess
Very common issue in 60's/70's vehicles
You have Spark...................but ONLY when starter motor is active
Run a temporary jumper wire from battery + to coil + and engine will start and stay running
Could be ignition switch under steering column issue
In the old days the coil would burn out faster if given 12volts all the time when engine was running, but coil needed 12volts to START the engine
So the ignition switch has TWO pathways to send 12volts to the coil
In START 12volts is sent to the coil directly(TFI), thru the safety switch actually
In RUN the 12volts passed thru a Ballast resistor(or resistor wire) so coil only gets 8volts
Ballast resistor was no longer used in 1985 but the WIRING for it was still there, no ballast resistor but TWO separate pathways, your RUN pathway is broken
Diagram here:
https://therangerstation.com/tech_library/images/durasparkwiring.GIF
The "S" on ignition switch sends 12v directly to the Coil +
In "R" position a separate wire is used, and this wire is broken so when key moves from START to RUN coil loses power
You may even have a ballast resistor and it is bad, google: Ford Ballast Resistor to see what it looks like
You no longer needed it so just remove and bypass it, hook the two wires together
It will usually be on the firewall, drivers side