I believe the once and rifle, always a rifle is a federal thing.Hi Kevin. Check your local laws to see if it is legal. I know in oregon if you purchase an AR in a rifle configuration, you're not supposed to convert it to a pistol.
As far as a conversion kit, check with anderson manufacturing and palmetto state armory.
In my opinion, keep your rifle the way it is and start a new project by piecing a pistol together.