I purchased a new set of axles for my 64 El Camino, after market with tapped and blank holes. They have the same number of splines as the originals, but will not slide into the gears. When I pulled the gear out and placed it on the axle, it looks as if everything lines up, but the splines are a little rough at the ends.
Should I have t "massage" the splines to make them work? I'm taking them back tomorrow, but thought I'd ask here first.

as far as I know you shouldn't have to touch the splines, they should be ready to install. I just installed a set of Superior axles and they slipped rt. in. Make sure you got the right axles or get them replaced.

I took the axles back to Original Parts Group, along with one of the side gears from the rear end. They didn't have any thing to match and didn't recognize it as being a Chevelle axle. The axles they had that the side gear would fit didn't have the c-clip grove in the end.
Where do I start? If I buy the (slightly) larger dia axles, can I buy new side gears that would fit the axles and the ring gear? I haven't looked for numbers on the housing yet.

I think the non "c clip" axles were from some Buick/pont/olds rears, I would call up Superior or Moser and find out exactly what you need,
you can get axles from either place for about the same cost as orig. replacements and they will be higher quality

Since you're in California, and So. Cal I'm guessing if you went into OPG, why not call True Connections? They'll definitely have the RIGHT part.

The guys at Original Parts group suggested I go to J&S Gear in Huntigton Beach, ...they'd know what to do. It seems early '64 10-bolt rear ends had 27 spline axles (although I swear I counted them more than once at 28), and were later changed. So, he sold me 28 spline side gears and new 28 spline axles.
Problem solved! Early '64s also have smaller rear UCA bushings at the rear end.