It's less likely for a catalytic converter to cause a misfire than it is for a misfire to cause a catalytic converter efficiency code, and possibly even damage to the converter if ignored for too long. I'd be more inclined that the cylinder misfire was what was causing the catalyst code, and eventually the misfire got bad enough to send the van to limp home mode and trigger the P0301.
Are you certain that the spark plug wires are good and securely connected to the cylinder #1 spark plug? If you pull the #1 spark plug now, does the color suggest it's burning rich, lean, or maybe some contaminant? Do you have access to a cylinder leak down tester? I think the 1998 3.0L has coil packs an not a distributor, correct? Could you swap coil packs around to see if the misfire changes cylinders?
-Rod