I just wrote a whole bunch of stuff and then it disappeared. I'm not going to write it over again! :-)
Anyway, one of the things is that there's a good possibility that a decent amount of HOs were sticks because it was a performance motor that could have been ordered by performance-minded people. Or, you could have a guy who wanted the top motor for his luxo-barge and wanted a luxury-minded tranny to go with it, in that case an automatic.
Visit
www.gmheritagecenter.com to see what resources they have. And
www.wildaboutcars.com is also a good resource for other stuff.