As I understand it, hydraulic cylinders are de-rated by 50% for pneumatic use, and pneumatics parts must be tested to 120% of their operating pressure - so you are OK in terms of static pressure, but when you use it with gas your cylinder will be moving much faster, and your cylinder will have to cope with the dynamic forces when it hits the end of its travel at high speed. Testing this is more difficult, perhaps someone experienced with high pressure pneumatics could help here.