Regarding the difference between the top and bottom bushings: bear in mind that the height of the bottom bushing affects the overall truck wheelbase, whereas the top bushing has no effect on wheelbase. This is all regardless of durometer/hardness. Durometer of course has no effect on wheelbase, duh.
Case in example: replace your bottom bushing with a taller one, and you'll end up with a (tiny bit, but still) longer effective wheelbase. Replace it with a shorter one, and voilà, the wheelbase diminishes.
The top bushing height mainly affects the turn and how flush the kingpin nut is (ie. the perceived looseness of the truck), but it does not affect the wheelbase.
In a nutshell: bushing height and durometer are completely different beasts. Hope this helps.