Yep, justin's right on there. Not that I dont have alternative reasons for you keeping the truck you have and parting it out, lol. But Justin hit the nail on the head about keeping the truck around for random parts you may need during the swap.
Besides, from what I understand, you know how this engine was cared for, and how many miles are truly on it, etc. You'd likely spend a ton more money on just an engine than to keep the one you've got. You'll rarely find a decent running 6.9 or 7.3 for less than 1500-2000.