What you are asking about is using Metal Techs Long Travel A Arms, they are set up for the Honda 450R shocks they are 16.500" over the LT500 14.375" shocks. I guess if you do some upper shock frame mods you might be able to run longer shocks.
You have the option of ordering these a arms for stock LT500 spindles or using the Z400 spindles.
In reality these are not really long travel arms just set up for longer shocks and will have the benefits listed that MD2 wrote.

To use a longer shock with the OEM arms you would require to make new upper shock mounts and would have to figure out where to place them and the angle geometry to weld them onto the frame, Glamisrider had a heck of a time doing this with set of YZF450 shocks, maybe he can chime in with some photos.