You can't really choose where to spawn. Not unless you were to write a custom script for pyspades like the
team survivor mode has. I don't know much more about that. But
this may be of assistance.
But you might find this graphic useful:
If there is no land at all, the intel and tent and spawns
will spawn in water. In a custom map the objects will always spawn in the same "random" locations. The game probably uses an algorithm which bases the random number seed on the map itself. If you use the standard "gen" maps this will always be different for each randomly generated map. But on a static custom map, it will always be in the same place. So you might be able to find just the right balance that would make things spawn roughly where you want, but it's hit or miss.