When we got the '66 Newport 2bbl when it had 7100 miles on it, when the new '67s came out, when I pulled the plugs out the first time, it had J-14Y Champions in it. When we got the '72 Newport 400 2bbl new, it had J-13Y Champions in it.
One reason for the colder plugs with the 4bbl motors was, as I read a good while back, was that the colder plugs would be less succepible to contributing to detonation at WOT on non-premium fuels (think law enforcement fleets which got their fuel from a common tank, usually "regular" grade). Which could also be why some 4bbl engines only had about 32 degrees total timing in the distributor, rather than the 38 degrees that the 2bbl usually had.
When I got my '70 Monaco 383-4 "N" car, I knew about the colder plugs for the 4bbls, but I also knew I would not be at WOT all that much. It had gotten a tuen-up at the local Chrysler dealer before I bought it used, with 83K miles on it. It had the colder plugs in it. In normal use, they didn't appear to be "not hot enough" to keep the ceramic clean. Ar that time, I would have suspecte that a plug 4 heat ranges colder would have looked different, but they really didn't. But then they were probably looking like a good "extended gap" plug should. Running hot enough at lower loads to stay clean (in a motor that was not running too rich and not using a lot of oil), yet being cold enough to not cause overheating of the tip and contribute to a possible detonation issue at WOT.
In general, it's been my observation that, over the life of the B/RB engines, the "high performance 383 4bbls" got the colder plugs from the factory (historically J-10Y Champions) and the "normal/consumer" 2bbls for the warmer plugs from the factory (historically J-14Y Champions for the pre-emission-era 383s).
Now, the term "High performance 4bbls" is most likely in comparison to the 2bbl motors. Rather than engines as the 383/335 vs the 383/325 engines per se. As the 383/330 "N" in a C-body had the 256/260 cam rather than the 268/284 cam of the 383/335. Check the SAE specs for where the 268/284 cam really was installed, as a Challenger 383 "N" would get the 256 cam, but a Challenger R/T 383 "N" would get the 268/284 cam.
Just my experiences,