Your dealer will have access to a JD document showing what all is needed and what lengths are available from JD. 

You need a foot/power harness on the tractor side if you don't have that already. Then a front extension harness from the rate controller pages that plugs into your open ISO tractor connector and a center extension if you need more distance back to planter frame. Implement receiver harness back at the 6000. 

As for length, you'll just have to measure the route it's going to take and go to the next longest length. Or order custom from AE as mentioned.