i believe its due to the fact that number 3 cylinder runs hotter than the others on a cossie lump. and for that reason with the bore being oversized and effectively thinner it causes the crack. if you can retard no3 a degree or so and add a splash more fuel it will keep it cooler.
nothing wrong with liners tho. some very powerful engines are liner engines. just as long as its done right. always hear about how bad the 205 block is etc etc. my opinion is that they are fine if done properly and mapped properly etc.