Hi Moose,
The naming 421 is to unspecific.
The axles of the 421 with 45PS are almost identical with 411 new axle style. The 421 with 52PS is different, i.e. bigger brake drums, not 350mm etc. The gear ratio of the new style axles 411 show up with different ratios. The last 522 manufactured 411 have the same axles as 421, like the swivel beraings in the front axle etc.
Never change axles without checking the same axle ratio for front and back axle, otherwise four-wheel drive will cause problems, Unimog 411 have no differential between front and back axle!
Markus