I know I am replying to a rather old thread but after discussion about sooty AB6 plugs in a LC 2 litre last year,  we are running fine on AB2s.  This might seem a huge jump but according to Green Spark Plug Co. it isn’t really.  Just keep an eye on the insulators to check they don’t then run too hot.  There seems to be a school of thought in some circles that modern fuel in old cars works better with a slightly hotter plug.