Valve Maximum Torque

740 psi is one hell of a pressure. Best suggestions can be given by Actuator Sales guys from whom you wish to buy since then, they stand responsible.Try out this question with 3-4 selected and take in writing with reference to their selected model.

The maximum torque that a valve can withstand is determined by that we call the "weak link" that is the weakest point in the kinematic chain.

The normal way to act is to calculate the torque necessary to act the valve in the worst design condition considered (system pressure, diferential pressure across the valve, flow rate, temperature, etc). This torque is independent of the type of actuator (manual, pneumatic, hydraulic or electric), then you must select an actuator with a rating greater than the calculated torque (do not use a safety factor too large because in the case of failure of torque or limit switches the "weak link" would break)

I recommend not to use safety factor greater than 1.5

Anyway, in a ball valve the valve rating (class 300) in your case is not related with the weak link. The weakest point use to be the mechanical travel stops that you need to fit the travel to 1/4 turn or the bolting between the actuator and the valve.

In more than 20 years I didn't find any valve of any type in which the weak point from the point of view of torque/thrust applied by the actuator were the valve pressure retaining parts which are those defined by the ANSI rating.