Look at the wear pattern on the cup. It should follow the same track all way round the cup. If you can see that the "track" follows an elliptical path, then the cup is not square with the cone.

Where did you get the spindles? NOS? Sometimes parts were never sold because they were defective and when sold as a lot the fact that they were defective gets lost. I have found ring gears with one bolt hole not tapped, connecting rods not finish bored and other weird defects. One spindle might be bent between the inner and outer bearing, which could result in bearing failure.

I recently ran across a heavy truck transmission that had been incorrectly machined from the factory, resulting in repeated bearing failure. Of course it was out of warranty so it cost $500 to have the machine shop bore the bearing bore square and sleeve it back to standard.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.