You can cut down your existing old and tired leaf springs to install a narrowed beam.  This can be done with a die grinder or chop saw.  Don't try it with a hack saw!  It's hardneded steel and will just tear up your blade!  You'll also need to drill the dimpled holes to mount the grub screws, which requires a special hardened bit and a bit of skill to get the location correct.  The better option is to install brand new leaf springs that haven't already been run for 40 or 50 years.  These are brand new leaf pack sets to replace sagging springs and provide more positive suspension response.  Link pin beams require (2) leaf spring packs of 8 springs each.  Ball joint beams require (2) leaf spring packs of 10 springs each.  Order (2) sets to replace all springs on one beam. Comes pre-drilled in correct location for mounting grub screws.