Here's the spline end of the original, pre 1981 stock long-side inner front GM Dana60 shaft. Prior to 1981 was when the shafts "necked down", which you can clearly see in the picture.

One other thing is well illustrated here. On an older axle shaft, even a 1981 and later "big" shaft, there will likely be a lot of wear and corrosion. this can be a significant weakness, as very often, when a shaft breaks, it does so at a place where there is a stress riser - frequently caused by surface corrosion, nicks, or flaws. That's why Drag racers polish their shafts to a mirror finish - to remove surface stress risers.

This pic of the stock Dana60 front 30 spline stub shaft also clearly shows where it reduces in diameter to about 1.258"

It also shows quite well the "bluing" marks on the body of the shaft created as the shaft passes through the coil in the induction hardening process.

Here's a comparison shot of the Dana60 30 spline stub shaft versus the 35 Spline Dana70 stub shaft. Note that both are exactly 12.0 inches long.

The other dimensions in the pic are:

Comparison of old and new long-side inner shafts:

Comparison of old and new short-side inner shafts:

It seems odd, and I have no hard factual or tech data to back this up, but the splines on the old shaft actually "look better" to me. I don't know if they actually are or not. But the real story is the dimension "E" - much larger on the newer shafts!

...as this picture and table illustrates. These are the long-side inner axles.

35 spline stub on the left, stock 30 spline on the right.

Reason enough to buy the kit right there!

Here are a bunch more comparison pics: