You should always replace the u bolts when swapping out rear ends. They have a tendency to stretch and get work hardened over the years. Axle tubes should be 3" on both. Make sure the spring perches are in the same locations on both.
So your saying 7-1/4" have 3" axle tubes? I wonder why the U bolts and shock mounts I had for a 7-1/4 wouldn't fit a 8-3/4"? I agree about changing old bolts.