Ford F-650 Owner's Manual page 142

Load Carrying
We do not recommend performing, or
approve of, suspension conversions.
However, we understand that, on occasion,
others install aftermarket add-on
suspensions on the truck chassis that allow
operator control for weight transfer from
other axles (such as air lift axles).
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I n f o r ma t i o n P r o v i d e d b y :
F650750 (TBC), enUSA, First Printing