30 vs 35 spline axles

They use the same bearing on the Dana 60 as the 8 3/4. The end of the axle where the bearing sits is the same diameter on both.
The end of the axle that goes into the differential is smaller on the eight and three-quarter than the Dana. Meaning that the 8 3/4 is a 30 spline and the Dana being a 35-spline.
As said previously you can go 35-spline 8 3/4 but you'll have to use a 35-spline spool to do it.
30 spline axles are so that you can use the Ford trac-lok and 8 3/4 axles in the Dana.