The Champions are what I ran last year. Just thought I'd see if there was something better, but at this point it looks like I have the right plugs in there.
I use the champs also in my 660 with Nitrous. No problems, worked great.
For the larger dose of nitrous I would cut the ground strap back to the center of the electrode.(Just to cut down on the chance of preignition or detonation).
Champs all the way for the liberty twins.
Never tried anything else but there was a discussion a while back on the
NGK Iridium BPR9EIX and someone stated the tech reps for NGK said not to use them. Although I see them reccomended for use on merchant websites.