I'm fixing to put a N20 system on my buddies 2003 5.3L silverado. It's an adjustable wet system 35-150 just seeing what plugs to use.

 

NGK TR55s are good for small jettings 35-75 HP but can be used for higher if properly gapped and frequently checked upon (gap 0.038-0.045.)

Autolite 5245s are good for the lower HP settings as well an are a good standard cooper core plug.

NGK TR6s are good for much larger jettings 100-150 HP but again can be used for higher when properly gapped and maintained (gap 0.035-0.040.)

If you are running stock coils and such the TR6 with smaller gaps may cause po300 MMF codes to happen on occasion when driving NA.

Just remember to use a non-platinum plug, get a plug with a gap nearest your desired operating gap (need the least amount of adjustment) and check them frequently to insure you are operating in the correct heat range and gap.

 

he is right on the non platium plug
as the tip stays hot and can cause pre ingition.
Not a good thing with NOS