remove the pump, you will see a small copper corkscrew vine that drives the cable. Otherwise a possibility would be to convert your rev counter to electronic, doing away with this stuff, and getting a FAR more accurate reading as well.
I have an electronic tach from a late spider, I was saving it for my TS project but I think I'm going mechanical. I'd sell it, if you're interested.
To drive the mechanical tach you need the reduction sprocket that is part of the timing chain system. You can get a suitable one from any 2-liter, maybe other engines as well. If you go this route, you'll need to cut it back a little in a lathe to get it to fit proper. Check the description on Jim stecks autocomponenti page describing the swap.