i would use champion QC57C race plugs. i have always used ford pp8 on mine at 330 bhp with no problems at all. then i built a new engine (450 bhp ) and it just blew the pp8`s up within 100miles ( litteraly ) 3 sets !!
the champs are cheaper too, £8.50 each, i have now done 1k on this new engine and has been as good as gold.
so it all depends on the engine spec.