No Spark, '84 258

Do you have voltage to the coil? Rats really like to eat early 80's wiring for some reason.

my best guess would be the ignition mod. but dont know of any tests

Looks like you have some good tips here!
The spark as you stop cranking is because the starter relay, (often called solenoid as it sort of is) supplies full battery voltage to the ignition system during cranking to compensate for the lowered voltage caused by starter draw. When you let up on the key, the plunger retracts, and breaks that connection, meaning there's a fast drop in the voltage to the ignition system, usually causing a spark from the coil. It won't necessarily line up with a particular plug, so you need to check it from the coil wire, not a plug wire.

I hate to say it, but I've replaced countless ignition control boxes over the years, for 1, my own 1984 258, and as a mechanic, many others prior to that. We had to keep them in stock at all times - Fords and AMCs with that system often lost the box. At times it was only when hot, we'd cool the control module/box and it would start and run. It would get hot and die again, and some had to be towed in off of I80 - died on the Interstate.

I don't have my Motorcraft stuff handy, so can't give any other tests at this time......
I also tend to think the ignition control module on the fender...... but don't spend the bucks based just on that hunch........

My son did the by pass on his YJ along with using an aftermarket module mounted in a small box that he bought. It is wired up so if something should happen with the after market module he can plug in the stock one. The by pass is in place for both modules and IIRC him telling me the ECM also retards the timing so he had to play with that but I did not pay much to what he was saying