Looks like the blade uses a pair of single acting cylinders. SO in you case it doesnt matter which line it goes on. Because one line is supply and the other is return when angling right. When angling left their roles reverse. But yes, get a bi-directional control and you can tailor it to the exact speed you want.
Just make sure you put the control on one of the lines going from the machine to that crossover valve. If you put the restrictor on one of the lines going from the crossover valve to a cylinder, it defeats the purpose of that valve.