changing rear axle

[JW76]

I have a 1969 12 bolt axle that I want to swap into my 1986 truck. The leaf spring perches are 45 inches apart just like the current 86 axle but the perches themselves are a half inch narrower than the 86 ones. Do I need to cut them off and put the wider ones on. In addition the shock mounts are in the wrong location. How should I go about this swap and what is the proper procedure for measuring the locations of the new perches and shock mounts and then welding them on? Thanks for any help.