Ignition switch tied to ecm?
 

Is the ignition switch programmed to the ECM, or are these trucks to old for that?
The reason I ask; I bought a wrecked 94 without a key and swapped over the ignition switch & key from my 94 gasser parts truck. It’ll fire right up, and then immediately die.
Only other thing I can think of is my brand new FSS could be bad out of the box?

The key is a "dumb" key - just a piece of metal easily duplicated at a hardware store/home store/whatever. Did you swap only the lock cylinder, or the actual ignition switch? If you swapped the switch itself, I'd suspect a wiring/connector issue first. It is also possible your FSS could be bad as you suggested - have you checked to see if you have voltage on the "hold" circuit when in "run" position?

I swapped the whole switch assembly. I’ll double check the wiring, and then check the FSS with my meter.
I was in a hurry to move the truck, and wanted to make sure I hadn’t overlooked something stupid.

Why not check the FSS first? That will tell you which way you need to work from there...