i tought these big JD engines Always leak water at the gaskets..
have seen a lot off leaking JD engines (8, 10, 13 ltrs) in forage harvesters and a lot of them have leaking head gaskets that are sweating to the outside.
I would suggest to buy a service manual (they are not that expensive on a dvd straight from JD) or just check the web, there might be a few around.
and this manual will coverage the whole tractor so might come in handy when you'll have something else with the JD.
set the valve a bit loser (0,05mm) then suggested, and then reset them after 50hours (if its easy to get to the valves) this way you'll know you didn't tightened the valves to much when the head gasket get compressed the first hours.
It's quite the work but nice to do:thumbsup
good luck