Looked my limited numbers of pictures with the area visible and from 5F1206xx + 5F176xxx I offer the following. Appears the location of the hole didn't change - well at least these two didn't on a manual car. On manuals the speedo cable enters the transmission tailshaft housing on the passenger side while automatics enter from the drivers side.
Automatics like 65 & 66 V8 cars have the hole (in the lower drivers side firewall) closer to the steering column hole rather than on the transmission tunnel area. In most cases the automatic hole was plugged with a rubber plug as used on other areas of the car body. green arrow in the later, 5F176xxx example
Now your question of if or when did they produce a firewall and install them without the automatic speedometer cable hole will take some more looking
Hope this helps