I tried the "move the pins, etc" to put a Vanagon tach into my 84, with
lots of frustrating and relatively expensive failures. I spent a lot of
time removing and re-removing the clusters and foils trying to resolve oil
pressure warning system anomalies and dealing with those minute
connectors.   I finally decided a small modern aftermarket tach, which I
mounted dead-center on the cluster in about 30 minutes.  I spent about $20
on that and it's giving good readings, though it IS just a bit too bright
with it's LED backlights.