Umarex 850 to hpa

Hello
i have a new Umarex 850 and a 13ci HPA tank (at 1200 psi), and adpater for HPA-Paintball tanks
But i cant install Because the tank is very wide, and i dont like to cut the stock in the forehand,
Im looking for a long adapter to connect airrifle – HPA tank.,
Anybody can help me?