On our 70 (as well as our son's 68)-----------------WHICH I BELIEVE TO BE CORRECT------------------ after the speedo cable comes through the firewall, and goes down under the car, it passes UNDER the front of the Muncie case, to a spot welded clip on the RIGHT floor and curves over the the speedo fitting on the tail housing.
There was a clamp attached to the left, lower bolt, attaching the Muncie case to the bell housing to retain the cable in place.

The first picture is our 70 conv. The next 2 pictures are our son's 68.
On our 70, I fabricated a second clamp for the bolt on the right side which tucks the cable up a little more, and further from the header.

 

Thank you so much for the pics and description!

 

I'm not 100% sure on this and I'm probably wrong but I believe on a factory 4 speed runs through the dash and out the passenger side firewall then down to the trans. When i converted to 4 speed, i ran it out the existing drivers side hole, over the bell housing to the passenger side and to trans. I used an rbc clamp to hold it on the right top trans bolt and zip tied it to the trans crossmember. I did loose the zip tie once and it rested on exhaust pipe just behind the collector and melted it. I now use two zip ties.. haha. I don't think i'd want to run it under the trans like that, run over something and it could rip it off easily.