I assume that is an extended reach plug in the zrf series? if so prolly try 4554 #8 race plug from NGK, they are cheaper at speed shops. $18.75 for a set of 8 here in jax at my speed shop I use.

2000 Dakota R/T, 408 magnum, 727, Indy heads
1000cfm 4150 carb, 93 octane fuel.
motor; 10.258 @ 132.78
200 shot; 9.262 @ 144.69
racemagnum