Larry -
Bummer! We have exchanged mails about your steering box rebuild. I thought all was well. These things don't usually leak where yours is leaking. I suspect that during the rebuild, a new sector thrust adjusting set screw may have been installed in an 85 year-old threaded hole. If it is slightly undersized, fluid will eventually work its way past the threads and start dripping, even a thick molasses like 600w. You might try removing the nut and wrapping a few turns of a heavy sewing thread around the threads near where the adjusting set screw ("plug") enters the housing. Perhaps coat the thread with a shellac or Indian Head goop first, followed by another coating with the sewing thread in place. Then carefully tighten the large nut. This will force the sewing thread against the housing body and reduce or even eliminate the tendency of the lube to drip past the adjusting set screw's threads.
Because the housing is full of lubricant now, removing the plug to coat the entire length of threads won't work. The goop will ooze out. Maybe you could sacrifice the lube inside by removing the nut and adjusting screw and allowing the level to get below the hole's bottom. Once drained as much as possible, jack the right side of the car up so that the remaining goop will flow to the opposite side of the hole. Then you could clean out the housing's threaded hole with alcohol or acrylic lacquer thinner, as well as the "plug" itself. Coat both with a sealant such as Permatex #2 and reassemble, adjusting the sector shaft's end play if the nut's position has changed or it was removed. Allow to set up for a few hours before putting fluid back in. You might try the fiber washer suggestion beneath the nut, too.
While you have the adjusting screw out, test fit it to see if it is actually too loose. If so, try a different one, preferably an original.
Marshall