Large or small tailshaft?

[Clsccmro]

I'm trying to figure out if I have a large or small tailshaft on my th350 auto tranny. And even more, what speedo gear I need to purchase to correct my speedometer reading incorrectly. How do I find this information? According to a chart I used from the chevelle forum I need a 20 tooth speedo gear, but that's only if I have a 7 tooth gear mated up to it inside the tranny apparently, so how do i find out if that's what's in my tranny without ripping it apart? Any info is greatly appreciated. Thanks a lot!