Hey Kurt, you're right; the new tach-drive gearboxes are junk. The way I've had luck with one, (after amassing a pile of failed units): Take the new one all the way apart. Lap each case half on a surface plate. Glue a thin paper gasket to one side, reassemble. Lube with JD Cornhead grease and reinstall. (I think I learned about cornhead grease from you on the SpriteSpot years ago in relation to steering racks...)
I think the new one's fail because a.) the mating surface on each half is not flat and b.) A few of the new one's I've opened appear to have a tiny dollop of plumber's grease inside. Once they get hot, the goo leaks onto the distributor and failure is near.