"I adjusted the ZF steering box to have very little play in it."

This could well be your problem. There mustn't be any obvious stiffness around the centre position. Put some stands under the front suspension A arms and check the feel of the steering. Adjust out until any stiffness disappears. Just being able to feel the centre position is a good place to start but you might have to slacken the adjustment off just a bit. The screw thread is very coarse so an eight of a turn makes a lot of difference. Better a bit slack than tight as too tight will wear out the box quicker. It's normal to have about 1 1/2" of play either side at the wheel, sometimes a bit more. The Pitman arm on the box must be in the correct position, you may have had it off. There are marks on the arm and the bottom of the shaft and these must align. This is the centre spot on the box and the box must be adjusted with these correct and the arm pointed straight ahead. A fraction of toe-in is good, as mentioned. Car should drive hands off at moderate speeds on a flat road.
It goes without saying that the box is topped up with the correct oil. SAE 80 is correct but using SAE90, the same as your back axle means you only have to have one can on the shelf. If the lower seal is leaking, fix it!