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+1, I'd start with replacing the HEP( hall effect pickup) in the distributor. Another thing to try: pull the rotor and sand the distributor shaft. On the carb motors, the rotor vanes ground through the shaft, if you have any rust or corrosion on the shaft, it can cause misfires.You can check easily if it is your ECU by simply pulling the vacuum line, you car should run on base timing. If doing this does make your truck run smoothly, then that's your problem. When you unplug it, your truck will run kind of boggish, but shouldn't die. Based on what you are saying though you have checked alot of things, but I didn't see the pick up coil on your list of replaced items. This would be a place I would definitely check.
These engines with a carb tend to be pretty cold blooded, so the stumbling until warm isn't unusual. The intake is heated by coolant, so it's not unusual for it & the carb to get pretty warm.
I also had an issue with my Scamp when I first bought it. It would bog, cut out, and run rough after sitting warm for a few minutes. It ended up being a bad ECU. It would heat soak, and act up. Replacing it cured the issue.