[Synchro909]

Many perches have been neglected over the years to the point where they are very badly worn on the under side. The bush doesn't contribute to the strength of the setup so if your perches are that bad that the shackles have left a hollow spot on the top of the axle housing (trumpet), you might be best advised to repair them. It isn't difficult. Find yourself a piece of copper or brass rod the size of the hole in a good perch. Insert it into your worn one and arrange for it to be held in the "up" position - the opposite direction to the worst wear. Now proceed to replace the metal that has been worn away with weld. The weld will not adhere to the non-ferrous rod. Once done, remove the rod and insert your new bushes.
I'm sure others will chime in here too.