Andy ,
Not sure there is a ngk help line down here .as a daily driver the -6 is fine especially with biggish cam and 4500 carb .
Dont forget we dont get oxygenated fuel here . Our octane probably equeals your 95/96 ?
I actually talked to ICE ignition here a few years ago and they recommended a -6 .
I have not had pinging or any sign of preignition . No matter weather or engine temp , long highway or city driving .
Its a bit like beer , got to try it to see if you like it

Tex