I just had to write about this... This very simple technique is still overlooked by some. It might save you a lot of "mounting" time when applied correctly. I personally find it very neat, so I took the liberty of combining this small "educational" video... (This is a very old video, which was filmed by a very old digital camera, many many years ago).
Note that it will not work for all pumps and motors - a series of conditions must be met. The pistons must move freely, the swash plate must be horizontal, you must be able to stop the pistons from rotating. The job will be much easier if the cylinder block has guided piston cavities, like the original series 90 blocks have.
The basic idea is to push gently the cylinder block in and turn it at the same time. As soon as you familiarize yourself with the technique, you'll see it's really easy. Enjoy!