The RC (reserve capacity) figure is supposed to be chosen by the manufacturer to protect the starter motor in the event of a non-starting engine. IE-it will go flat before the starter motor burnsout / catches fire if the driver just keeps cranking the engine.

Congratulations, sir, on finding, for the same physical size, a battery with a slightly better rating than the original! No limit.. More reserve capacity, more amp hours, the better. Go for it if you have to fit a new one! However... have you got a battery which will work with Ford's 'Smartcharge' system? The answer to that will be 'Yes' because you put in your car's registration! Ordering new bits is so easy these days!