It's not going to help you much, but my Veglia Tach works perfectly with my Duraspark II (Non-California, Blue Module).
As your review the Duraspark Diagram - it doesn't show a Tach Connection, but a look on the Pantera Schematic (Late-L) shows it coming off the side of the ignition coil that connects to the distributor (points side). The Tach is powered by +12V. It looks like there is a ground there too.
I am sure my Duraspark configuration did not modify that portion of the wiring.
I attached a drawing of the Tach Circuit for you.
I am with the others, who seem to be leaning toward a bad Tach.
There is no mystery - if you can borrow one, just make three alligator clips to the tach connections... Clip #1 to (switched) +12V on your bulkhead, and #2 to the (+) side of the coil, and #3 to ground - then start the car. Compare it to your dash reading. This would pretty much be a two minute check.
Or pull your tach, and send it to someone who has the capability and expertise to test it (I am not being facetious here - that's a good suggestion, and if my test shows the remote Tach works fine - then that's what you are going to end up doing anyway...)
Good Luck
Rocky
NOTE: THERE IS AN ATTACHMENT BELOW THIS... YOU HAVE TO CLICK ON THE WORD "ATTACHMENT" TO SEE IT.