Are you SURE that that is the size that you are working with? I have a I7 980 with 24gb ram machine running 64 bit windows 7 and just running the clouds took it enough time for me to have a water break...

On to the question at hand, are you starting with a sketch, or are you using the clouds/threshold to define the coast? If you start your drawing at the dimensions you want (60x40 @ 300dpi) the resolution of the threshold should be at the resolution that you need. I am not sure of the mods you made to the threshold setting to get that image, but when I did that at those image settings, the image was pretty well defined between the black and white......