Any suggestions on how to get the ujoint off the axle
Slide hammer with a hook on the end?
It shouldn't be that hard to get off. And like Jerry said, it needs to slide fairly easily on reassembly, so any burrs on the splines will need to be dealt with.
The procedure for adjusting the bell is outlined in the service manual. See page 7-11 of the 48 to 51 Chevy Truck Shop Manual. The process is the same for later years.
