This is what I am referring to. The only way I see to access the ball joint plug is from the top thru the hump cover.
The ball joint fill plug should be accessible from underneath, using a lube pump. Too close to the cab floor to be able to pour from a bottle. There's a bearing separating the rear extension of the transmission from the main body. The shop manual calls for 1-1/2 pints in the transmission itself and 1/2 pint in the rear extension for the u-joint, so 1 quart should do the whole thing, assuming you don't spill any.
Jerry's idea of a square section rubber ring is spot on. I may even replace the new cork/rubber packing ring that comes with the repair kit in mine, as I haven't installed it yet.
