I did the same swap, '68 to '72 axles. They '72's are longer, but the bearing surface is also longer so it mates with the bearing no problem. I think that the bearing surface is the more important than a little extra length. Anyway, i am running them as is, but haven't had a chance to put alot of miles on to test the theory. I'll see if i can find the pic of the two axles side by side i posted here a year or so ago.
RD
Here it is: '72 on left...
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