Is the spring plug stuck within or against the barrel bushing?
Will the spring come out, spring plug and all?

• Sometimes you can turn the barrel bushing "through" the spring's coils, to the point where it's free to come out. That should free-up the spring and its plug.
• The plug is attached to the spring. The end of the spring is a prong which fits into a small dimple in the plug. "Unscrewing" the plug off of the spring might remove it.
• Worst case, put the spring and plug back where they belong, and swivel the barrel bushing back into place. Now, pushing the barrel bushing and the end of the slide that's below the barrel against a table top will allow you to remove the slide stop. Carefully releasing the table's grip on the slide will allow you to remove slide, barrel, bushing, spring, plug, and all, all as one unit. Carefully lift the rear spring plug out of contact with the barrel, release tension on the spring, and you should either be able to complete disassembly, or to find the problem.