If you want to save money and seeing as you don't appear to do your own wrenching, sell the 6.0 private party.
If it's a 80kmi cherry grampa driven truck, I'd say you could get $15k all day or maybe up to $20k if you find a buyer uninformed about 6liters.
Take that $15-20k and buy, private party, the lowest mileage, newest best condition HD gasser truck you can find from any of the big 3 with the nod going to GM and Ford over Dodge, unless you can find a 4th gen Dodge Hemi.