I had a 70's something Evinrude that I bought brand new and it came with surface gap plugs. That motor never idled very well and usually ran like crap at any speed. A few years later I took it to the dealer for a tuneup and when I picked it up the mechanic showed me a factory service bulletin that recommended a change to a gapped plug. That's what he put in and it ran better then.
Not saying this is the same issue, but maybe you should check the Chrysler archives to see if such a bulletin exists. Just because the factory manual says one thing doesn't mean it wasn't superseded.
On another note, I once had an Evinrude kicker that fouled plugs like there was no tomorrow. At the advice of a factory engineer I switched to Splitfires and the fouling was eliminated 100%. I realize many "experts" poopoo Splitfires, but it sure worked for me.
I have heard others poopoo ALL Champion plugs, but you could have fooled me.
Bottom line is that sometimes it takes experimentation...
