John, I have heard that "general rule" before and always thought it was true but when I worked as a parts manager in the late '70s and General Motors had a choice of 19 engines in their vehicles in 1979. So for some of the least common engne sizes that required a different type of plug, rather than go to the expense of making up the plugs and inventoring themselves, they bought them from Champion, packaged and numbered in a AC Delco box. These plugs did not have the three green stripes around the porcelain body. I got this info from one of my friends in purchasing when we were having backorder situations at the time. Even the box stated "Packaged for AC Delco". Some more trivia that you might already know is that "AC" stands for Albert Champion??
But like you said the best way is to run your plug for a couple thousand miles and pull them and read them. They will tell you if you have the correct range and setting.
