The tachometer is electric, and an adapter is stupid simple.
Go to this site:
http://www.ashlocktech.com.
The product is Tach-Adapt. It changes the timing of the tachometer, and can go from 4-cyl to 6-cyl. Lots of Porsche guys use it when converting a 912 to 911, and 914s to 916s. They are $65.00 USD.
I have one for my NOS GT Instrument cluster for my 164, but have not installed it yet.
David Riedle
USA