Ryan H, it should still be black/purple all the way to the Tacho.
It's confusing because it goes through the 14 pin connector twice. The numbers in bracket refer to the diagram.
Distributor(5) ->connector(84) ->speed switch (11)
Speed switch (11) ->connector(84) -> tacho (off page label 43 and 67 on part 1 of the diagram)
The two black/purple wires must be joined at the speed switch.
Here's part 2 of the diagram which shows the routing. It is for a '71 but I'm pretty sure the '70 is the same.
I'm happy to help you out.
[I will just get a bit of promotion in here though. Although our CDI+ has a higher ticket price, it is designed to work with all the original Porsche components including the tacho straight out of the box. If you value your time or pay a garage for install, our units can actually work out cheaper.]