I don't know of anyone that currently offers a short throw kit for the Spider. If you do.... I would love to see it or meet the guy. that is illustrated by the middle example in your diagram.
I have a shorter shifter. (the right hand example is my shorter shifter)
The stock Chrome shifter is approximately 7" long and is mounted to the shift lever with a nylon & rubber set of bushings and clips.
My shorter shifter is roughly 4" long and is mechanically fastened directly to the shift lever. Rather than the shift knob sitting 3" + above the tip of the shift lever, my design puts the shift knob less than 1" above the shift lever. the only thing shorter than that would be to mount a shift knob directly to the shift lever.
therefore, as your illustration shows, the travel below the pivot point does not change from stock setup, because the shift lever is the same.
I love your drawing by the way. Did you do that, or copy it from somewhere? Send it to me, will you?