These perches, designed by us, have a machined hole in the center so they fit over the existing perches on your 10-bolt or 12-bolt rear end without modification. These perches then need only minor welding to fix them to the rear end axel tubes. A 1" weld on both sides of the housing tubes is sufficient if your existing perches are not damaged or rusted. This can easily be done by a qualified welder in about an hour with the rear end in the vehicle. Moreover, if you ever wish to restore your vehicle to original GM specifications for collector car purposes etc. these perches are easily removed. Our spring perches enable you to use the wide range of readily available, custom coil springs that are pig tailed on the upper end only (factory style springs for 1968-1972 A-body chassis are pig-tailed at both ends). Without modified perches your choice of rear spring ratings and heights is severely limited.