There are two alternatives to consider for this: a hydraulic cylinder driving an arm which tightens a fastener, or a vane type hydraulic motor. Both have advantages and disadvantages.
1- The hydraulic cylinder is simple to implement, and can directly indicate the torque applied as a function of the hydraulic pressure applied to the closing side of the cylinder multiplied by the area of the piston minus the cross sectional area of the rod. However, being a linear device for the calculated value of the force applied there must be a right angle from the arm to the rod at the desired torque (sine 90 degrees = 1). Any other angle will introduce a sine error, producing less torque than the right angle. As it approaches ninety degrees from the ideal right angle the force will decrease to the point of applying little or no torque to the fastener. There are ways around that, particularly by using a chain pulled by the rod engaging a quadrant sprocket turning the fastener. This complicates the design as to recycle the rod to home would require a return spring, the dynamic rate of which would have to be subtracted from the calculated torque value.
2 - The vane type motor advantages are significant, particularly the force applied does not change regardless of the position of the driving axle. That, and it can drive the fastener from one thread to seated, then torqued in one operation.
The disadvantages are vane type motors do not take kindly to low speeds, and stalling can be damaging to the vanes and rotor. Commonly, this is worked around by introducing a gear set that allows high speed from the motor with a considerably lower speed at the business end. In other words a production gearhead vane motor. This increases the initial cost considerably and usually requires frequent maintenance or an overengineered gear set. Another disadvantage is there is no way to determine torque by the simple method used by the cylindrical type due to the constantly changing volume of the motor, though tracking the pressure and flow of the fluid may get it in the ballpark. Repeatably accurate torque values are a production necessity, and to that end a load cell placed into the output shaft line or on the motor housing, which would require the entire drive assembly pivot on it's output shaft centerline, with a computer tracking the output and actuating a PWM phased vent solenoid at the desired torque would do the job, again adding complication and cost.
There are other ways of doing this, but for the moment IMO those two are the best alternatives.
Rob
