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NAPA can sell you a high dollar stock Ford replacement, or they can make you one.
I opted for make me one, I ran individual 2/0 wires from each battery to the starter.
Cost was about 70 dollars for both. Measure carefully if you do this, and put some heater hose around the drivers side cable that crosses the radiator support.
Cheapest place I have found is your local Big Truck dealer or Interstate Batteries store... I had mine build at a Kenworth dealer and they were fairly reasonalble. About half of the price Ford wanted for them.....