Spark plug question

The heat range, is just that.
The chamber, with the correct AFR, will run cold at part throttle, and get hotter the harder that the engine has to work.
So the plug has to be able to survive for the full range of temperatures that it will be subjected to.
For a 4-bbl engine, that is able to work harder, you gotta install whatever plug it takes to not overheat it, which usually causes pre-ignition, leading to detonation, leading to engine self destruction.
A stationary engine running at 3600 would require it's own heatrange, versus your car, versus a race car.
But too cold leads to the plug fouling and then it misfires.
Hitting the window can be a chore.
I don't think there is any performance to be had in the heatrange of the plugs, unless it corrects for other conditions, such as misfiring, burning up prematurely, or causing detonation.

I have had the best success with Champions.
I agree with Toolman as to 12s in your combo
Coppers seem to run forever; I finally changed the ones in my 367 after over 100,000 miles since 1999. It doesn't start or run any different.
If you are running headers, the 5/8 hex can be a Godsend.
But you will have to pull a few of them, after some miles are accumulated, to see how they are doing.