Check out www.rustboy.com , and in his "making of" section, he goes over how he created his puffy cloud shots using a compositing technique called multiplaning.

Using multiple layers of cloud imagery, with lots of transparency, you can achive great clouds with minimal fuss, and fast render times.

Here is his movie about his clouds: https://www.rustboy.tv/movies/cloudlayers.htm

The best part, is if you want to create the "god ray" effect with this technique, all you need to do is check the docs under light fog, and if you place your fog-light above and behind your cloud-planes, just turn on raytracing, and crank the rays (ray limit I think) to 3 or more, you should get some decent results.

Most rays like that are done in post anyway

Good luck!