There are several positions of the tongue that will allow the overblow to sound in hole one, but most of them are not stable.
In my opinion the best way to play this is to put your toungue on the bottom of your mouth and try to get the tongue as low as possible for the entire length of the tongue.
The feeling is a little like having a hot potato in your mouth that you are trying to cool down by blowing past it.
Choke the blowreed with the tongue in that position. If you are able to get a relaxed but solid choke on the blowreed, you should be able to play a stable overblow.