This is what I am using:
NGK 7173 R5672A-8 Racing Plug $2/each.
Heat range 8
https://www.ngk.com/product.aspx?zpid=10047

I don't know about the fuel over here, but I think your pushing the limit?
The Stealth heads don't have as good a chamber and spark plug location as something like the smaller chamber Victor, Trick Flow, and B1 B/S heads.

Cool air into the engine and cooler engine operating temps (160-180 degrees F) should help