It's possible that the hotter plug caused a slight overheating condition which resulted in vapor lock, stalling the engine. But otherwise, about the only way a too-hot plug stops an engine is by provoking a seizure or burning a hole through the piston. What did the new plugs look like, when you pulled them from the stalled motor?
BTW - was this all done in a test tank, or out on the water? Sure, these are water-cooled motors, but they can get a lot hotter 'under the hood' when sitting in still air - and that's where the vapor lock originates.
Finally, for the most part, plug cross-references are general guides and some types / makes of plug just seem to work better in certain motors. As you might have noticed, NGK is my go-to plug - they've never done me wrong. And you should +never+ use any other plug (except for perhaps a Denso) in a Japanese motorcycle or car. However, the Denso u-groove plugs burn extremely well in the older Ford L6 motors, 240 cid, 300 cid.
I've never had good luck with Bosch plugs, except in BMW motorcycles and one particular moped. Champion are OK for lawn mowers, snow blowers, outboards / power equipment and.. Harleys. Otherwise, I avoid them like the plague for motorcycles or cars.
When NGKs are two bucks, and widely available, I just can't think of any reason to mess with anything else, really. Maybe try a pair of B7S some time.. or B6S if you want a little hotter, B8S for a little cooler.
While totally unintentional, I can be somewhat condescending. (That means I talk down to people.)