You can't use a DAM head on a non DAM machine. You can go the other way but not the way you want. Just go to a bone yard and buy the DAM harness. It is the same for JD 6620,7720, and 8820. You need the control switch for the cab. The control box that goes buy the hydraulic reservoir. Then the harness from the head to the control box and to the control switch.
It does not take long to install. I used to do it in under two hours after I had done several. They DAM is worth having it works much more reliable than the old switch type.