I am a bit late to the thread, but I do have one thing to add.
Before I do though, Nice work. And nice find.
Now what to add,
You really need to look at putting in a crossover relief valve between the two cylinders. If you havent already. With your current set-up, If you hit something with the leading edge, that fluid has NO where to go as your remote valve isolates that side of the circuit. It dont take much force at all, and you will pop a line or cylinder. ESPECIALLY with a plow that wide (with wings) and a tractor that big.
When that plow was mounted on the truck, there was a relief incorporated in with the pump. You have taken toat out of the equation. You need a valve that will dump fluid from one side of the plow to the other in the even you "over-pressurize" one side by hitting or pushing on something too hard.