need more octane in the fuel to handle the boost under those conditions... you have a roots type charger on that car it can give boost at any RPM problem is we dont have high octane fuel from the pumps in SA so detonation at low rpm and wide throttle openings is to be expected...
try some octane boost 90% it will fix the problem unless there are other problems at work here...
I carry the full NF range if you interested

it will stop the knock...