Gasoline octane here in the states is determined using the (R+M)/2 method. Not sure how that relates to your fuel octane rating of 98.

Like Lew said, I dont think that our premium fuel (93 octane) would be able to support that small cam at that comression ratio. What is your cranking cylinder pressure, in psi if you would.

A couple of things to look at or try.

First, Usually detonation is observed at lower rpm and higher load and may continue through the upper rpm range. If you only have detonation above 4800 rpm, that can be from a lean condition due to low fuel pressure.

Try spark plugs at least two steps cooler and a 160 degree themostat. Make sure cooling system is good overall.

64s center and 0.089 inch dia. plate restrictions in the outboards will put you close if everything else is unmodified, like the MAB

I think that 6200 rpm is optimistic for that cam