Can't you just get 98 octane anymore in Poland? Especially with 11:1, I wouldn't even consider 95.
Here in the Netherlands 98Fuel is still readily available.
95Fuel is considered partswasher here, albeit an expensive one

I have similar problems with my 496" stroker, 84cc Eddy heads and dual longram-intakes. 10.5cr.
Engine pings vigorously the moment you put some throttle in it and the rpm get up to 2300-2500rpm.

In my case both carbs are placed in pretty much the hottest corners of the enginebay, right above the exhaustheaders left and right. Once the underhood temps get hot enough one has to feather the throttle to prevent pinging.

Maybe your friends car could use some cold-air intake system, just like mine needs badly.