Tony, I think you will find that most hyd top links have 300-350mm stroke, but thats really not very relevant. measure the length of your current top link at its max length you would want it to be and then select the top link that is that length extended.
the lengths are altered by buying differing screw in ends.
Buy the larger diameter ram: the small ones are too quick moving, and the rods have a tendancy to stretch with heavy gear on the back, which makes them thinner with resulting seal failure.
Its also a good idea to have one with a check valve fitted, this helps prevent the ram creeping and is safer if a pipe bursts.
Dave
