Not to butt in - but - I have built my fair share of Muncies and have found that the Japanese bearings were actually some pretty good units.
More likely what has happened is that one or more needle bearings inside the input shaft that support the front of the main-shaft has dropped out when the transmission was assembled. It happens very easily! They will shake like crazy if put in the car this way. The real bad part is that it will wreck the transmission case if driven like this for very long because the front bearing on the input shaft is carrying too much of the load.

I typically use thick black wheel-bearing grease to hold these needle bearings in place while assembling the box.