If you're lucky or diligent you'll find which cylinder is likely to knock 1st (probably 5 or 6 by the water circulation, but some other feature can change this such as smaller chamber, shrouded exhaust valve, worn lobe etc.).
That one has to be protected even if it means running the other 5 too rich and retarded for best power - unless you'd like to try popsicle sticks and dams in the manifold?
