Manage your expectations. Just imagine if you left an automobile sitting out in the elements for 40 years. Even if you dropped a new engine in it, it isn't going to be safe to tour the country.
Use the checklist mentioned above, but you can expect that the axles will need to be replaced, almost by default. Tires should be replaced before you even move it. The floor is likely to have rotten spots in it, expecially in the rear. There may be rear-end separation, but you may be spared this as it doesn't sound like it has moved much. Even if the rear end isn't separated, if the floor is rotted out back there, you might create the separation if the rotten floor goes unaddressed.