We needed 500 ton clamping force between two large plates.  We modified eight 60 Ton hollow ram hydraulic cylinders to accomplish that.  The modification involved machining a different saddle to allow the cylinder to be held to the stationary plate and also replacing the bottom plug with a one threaded for a 2" threaded rod.  With all eight cylinders mounted around the circumference of the stationary plate the rods would go through the holes in the moving plate allowing to clamp a stack of hydrolysis cells between the plates and compressing them with a constant force for testing.  The fixture was located in the laboratory environment and was not used much at all before the program ended.  Those cylinders are great whenever a high force clamping is needed.  I am aware that my description might not be clear so please email me any questions you might have.