You can do a subset like you are doing. If everything is working ok after step 3, but mapsource crashes after doing step 4 then you made an error in that step. The key points are to remove the definition areas from the original overview map, then add the new definition areas. The most likely error is you didn't add the new definition areas.
Thanks RW. You were indeed correct. I missed the step where you had to delete the definition areas. I selected them, just didn't delete them.