Glad ya like the Cabin Set :-)
Ya have the right Idea, perhaps try some different cloud maps they may give you better results.
Also you can try putting the clouds on a single cell prop (just a big square plane) and use a trans map to make a floating cloud prop. It will cast shadows like any other prop so you can remove the node links. this may give you better shadow results if you use firefly and be sure to use "ray trace" shadows with a nice softt edge of 2-5.