1) Generate a density/probability mask
//generate 35:5 perlin(11111111, sin(random() * x), y, sin(random() * z), .35, 2, .0001)<.5
2) Scatter single blocks, the mask defines where those blocks are more likely or less likely to occur.
//gmask >35:5
//generate 35:13 perlin(randint(10000)...