ECU isn't part of the spark system as far as starting, just FYI, it "suggests" spark advance and retard info to the TFI module via the SPOUT connection but that's all it does for spark.
TFI system is self contained, good read here on that system: Ford EEC-IV/TFI-IV Electronic Engine Control Troubleshooting
TFI system has 4 components, distributor, TFI module, coil, wires.
In the distributor is the hall effect sensor that creates a pulse for each cylinder lobe that passes, TFI module reads the pulse and sparks the coil, spark runs back to distributor and rotor sends that spark to cylinders spark plug wire.
So it is the same as a points type system but no points to wear out and adjust all the time.
The hall effect pulse is passed on to ECU so it can time fuel injectors, so if there was no spark and no fuel(on spark plug tip) then could be TFI module or hall sensor issue.
The "letting go of the key" stall was a common issue on older systems.
There was a ballast resistor that was used for the Coil that reduced the "running" voltage of the coil to 8volts to make it last longer, during starting the coil got full voltage for hotter spark, but when running the coil voltage was reduced because hotter spark is not needed.
So when key is in START position the voltage to coil is traveling on a separate wire, than when key is in RUN position, so if engine starts but as soon as key goes back to RUN the engine dies then voltage to system is being cut.
You will see that in the TFI link above, separate START and RUN wires on TFI module wires
You have the problem in the RUN circuit, could be in the wiring or even ignition switch.
With key in RUN position check for voltage at the coil's + terminal
No, I haven't seen a DuraSpark or HEI conversion for 2.9l, could probably be done but fixing TFI system would be cheaper