Spark Plugs for LA w/magnum heads

Hi guys, I was just doing some research on plugs and starting to think I have the wrong plug in my engine. I have a LA360 with magnum heads and running Champion RC12LC4 which are the correct plug for magnum heads, but cant find any heat range info. I based this off of a 98 Ram with a 5.9. If I understand correctly I have to run that style plug because its a bit longer to get into the combustion chamber.

Since I could not find any heat ranges with Champion Plugs I went NGK's website and got info for a 98 Ram and a 74 Dart. Using NGK's website I put those numbers into Summit racings site and compared plugs. Looks like the Ram has a heat range of 6 and the Dart has a heat range of 4. I dont know how much this affects the car but I want to make sure I have the correct or best plug in my car.

So now Im wondering what plug I should be running in my car, or what are you guys running for plugs that you found work best for a LA engine with Magnum heads??