The key words here is You HAVE TO GET ALL THE OLD COOLANT OUT, So you either need to have it flushed. Or you need to pull the lower rad hose. drain out as much as you can. Fill it with plain water. Get the truck to opperating temp and drain again. You need to do this a couple of times. If you mix dexcool with green you will get sludge.
^no just pour it in. I would say, flush the system and go with the ethylene glycol (green) it is a lot better and dex cool is hard on seals. ethylene glycol will kill the family dog, it takes a lot more dex cool to do the same.
Use Dex cool only in a D-max , Never ever use other than dex cool and never mix ,don't listen to anyone that says different !! There is a reason GM uses it , I can't tell you the details I am not smart enough but I am smart enough to use DEXCOOL !!!