You need/should run 2 steps colder for nitrous.
especially when your first starting out.
one step colder plug is a good mod on a mark 8 because it makes the car a little more knock tolerant
with a colder plug you get a little more knock tolerance without giving up any performance.
AWFSA 22 C (one step colder)
AWSFA 12 C (two steps colder)
motorcraft PN's but good luck getting anyone to "admit" they are available.
Lonnie sells them at BOC
note the "C" in the PN designates a copper plug.
FWIW: I tried NGK plugs ONE TIME and that was when my car "launched a spark plug".
(not saying it was NGK's fault, but wasn't a risk I was willing to take a second time)
Many ford lightnings have also "launched" NGK plugs, ( I'm just sayin)