To just change the possible spawn points, there's a spawnpoints.xml under the GeneratedWorlds /<WorldName> folder. I haven't messed with those recently, but I think they should work.
(Take a backup of the file first, just copy paste)
Editing in new coordinates, and removing most others should work.
<spawnpoints>
<spawnpoint position="-727,44.36251,-1201" rotation="0,129,0"/>
</spawnpoints>
Three numbers, the middle one is "height", make it too small and you should be buried, but will be "auto-resolved" to surface. Make it too high and you'll fall into a start with a broken leg, or dead.
First number is "East", bigger the number, the more east. Zero in the middle of the map. Last one is "North", same deal.