I found a pipe that the axel shaft would fit in smoothly, set it against the floor of the shop (with a board between it and the pavement) and just picked it up and smashed it down until the shaft popped off.
On the long shaft it didn't take much time at all. The short one took about 15 minutes for me, and 30 minutes for a buddy. I guess it depends on how strong that snap ring is.
When I put the other shafts together, I just put the snap ring on the Calamini axels, lined up the birfield, and slammed it together. I have heard a press works well for that though.
You might look at just upgrading the entire kit though, including the birfield. Someone released new axels and birfields, but I can't find the reference to them now....
[Edit] Found it.
CV Unlimited. Has anyone else tried these out?