Fake it if you can't make it
It's usually not my credo but there are certain situations which you cannot bend enough to reach your goal without the help of post-processing.
I recently bought a HiTech big-stopper ND filter that blocks 10 stops of light. I originally opted for the Lee version but it seems to be impossible to get a hold of that filter if you don't want to wait for a couple of months - I'm not a very patient guy so I got the cheaper, lower quality HiTech versions for 10 and 6 stops. If you want to create long-exposures with very smooth looking clouds you need at least a 4-5 minute exposure time and on a sunny day, you will have to put 16-stops of ND filters in front of your lens.
That's exactly what I was planning to do the other day. I recently bought Joel Tjintjelaar's new Video Tutorial in which he explains in-depth how he creates his epic black and white long-exposures from shooting to post-processing.
So, I was setting up my tripod and camera in front of a office-building complex in Zug, Switzerland and I had this great looking streak of clouds just atop that building perfectly aligned. It took me about 5 minutes to align the camera and just about when I pressed the shutter on my remote-release, a car drove by and wasn't able to pass - when I was setting up I didn't realize that cars would drive on that sidewalk but I was wrong. I thought about talking to the driver as I had 4 minutes left for my exposure to finish but when she started honking and waving I thought: "Screw it, let's let her pass!"
Once she passed, I started again from scratch and a few minutes later I was ready again. BUT, the cloud was gone and all I was left with was deep blue sky. Great!
I decided to move on and find another location but due to the fact that all the clouds have vanished, I called it off.
Fortunately, I shot a few test-shots without the filters on to test the exposure and composition and decided to test a few things with those pictures when I remembered the "Motion Blur" tool in Photoshop.
I will not get into details on how I achieved the black and white final version - consult Joel's linked video tutorial mentioned above.
Step 1 - Remove the Clutter
remove the leaves in the top corners using Content-Aware Fill.
I tried to avoid the leaves but my first priority was the perfect angle of the building and I knew I could easily get rid of them using Content-Aware Fill.
I've used the Polygonal Lasso Tool to select the areas around the leaves. I then clicked on 'Edit'->'Fill...' to get them removed. It worked perfectly here as expected.
Step 2 - Stretch the Clouds:
After I straightened the top of the building, I've used the Rectangular Marquee Tool to only select the sky. While the sky selected I've added a new layer by pressing 'Command'J'.
With the sky still selected (this is important), I chose 'Filter'->'Blur'->'Motion Blur...'.
I then had to play around with the settings and they will vary based on the kind of clouds you have in your picture. For this particular sky, I selected an angle similar to the alignment of the streak of cloud and a very long distance of '1500' pixels to really stretch it out.
It is important to have your sky selection active as the Motion Blur tool would otherwise blur part of the sky into the building or the other way around and you don't want that.
What followed next was the conversion to black and white and all the other magic Joel teaches in his tutorial.