[Pete]

Finding the right spark plugs for a new engine combination can be time consuming and a bit costly but it needs to be absolutely right if the engine is to run up to it's capability.
Before anything, they have to be the correct reach. That means the plug should have the same reach as the threads in the head minus the washer thickness so when installed there are no threads showing either on the plug or the head and the electrodes should not be exposed so much as to be touched by the valves or the piston in the case of an OHV engine.
If in a preliminary test the plug is short of the head surface,
you will need a longer reach plug.
If the plug threads go past the head surface, you either need a shorter reach plug or a SOLID copper washer of correct thickness.
The washer that comes with the plug usually is a compression type and is adequate for stock applications only.
If you want a bit more than stock, solid copper washers will give a closer "reading" and work better longer. It has to do with heat transfer. A plug is designed to be installed with all of the threads in contact with the head so stacking washers is not recommended. Most of the heat transfer is through the washer seat, not the threads.
In some cases if the correct reach plug is not available, one thread in the head can be chamfered away so it doesn't show.

After the engine is installed and running it can be driven as it is intended to be and the correct heat range can be determined. This is the expensive part of the project. Sometimes it takes 2 or 3 sets of plugs to find what is correct.