So, drop your atmo quality samples to '16' and the cloud quality to '1'. When you've done that, visit the render tab and click 'edit sampling'. Select '1/4 first samples' or, just uncheck 'customise sampling', both will have the same result, you'll notice a massive increase in speed. This might be a bit grainy but just increase cloud 'quality' by '0.25' to '1' incrementally until it looks smooth, you really shouldn't need '50' for this, maybe 4 or 5 in the quality field, maximum.
I changed a few things in the cloud/fractal nodes here, mainly removing the first fractal and the multiply node, you can get a decent mask by using one big fractal and then apply that as a second, smaller fractal's blend shader only. I'll upload it once I've fixed it up but start off by doing the above, test renders will be a lot quicker and easier that way to begin with.