You won't be able to get enough leverage on it to do much good turning the driveshaft, but use high gear if you have to do it that way. A better method is to drop the inspection cover at the bottom of the bellhousing, and use a big pry bar on the flywheel ring gear teeth. That gives you much more mechanical advantage than a pipe wrench on the u-joint. Less chance of damaging something, too. If the diesel has been in there that long, it shouldn't be too hard to break loose, unless something is really badly wrong. You might find that the cylinders are too badly rusted to run anyway, as at least one cylinder would have had the valves open, exposed to humidity. I got into an Olds 330 engine once that was pitted badly after only 3 months of storage, and that was in central California. One cylinder had pits so deep that the cylinder had to be sleeved, with no damage to any of the others.
Try some Kroil, if it's really stuck tight. That stuff is fantastic! I've poured it into rusty cylinders, and come back a week later, and drained it out of the oil pan.
www.kanolabs.com Jerry
