Reduce leaf spring travel?

["ambradley@frontiernet.net" <ambradley@frontiernet.net>]

Before I throw money away, I want your edumacated opinion on whether
it's possible to stiffen the stock leaf springs to reduce the amount of
up and down travel.  Short of replacing the springs (or
adding/replacing a leaf), there are three products that claim to reduce
spring movement - clamps, torsion bars and helper springs.  Would any
of these have an effect on the amount of suspension travel, esp. down
(i.e. bottoming out)?