I cant speak about the lat model 120Y's but most of them had square instruments, while 1200's had round ones, so switching a 1200 Tach into a 120Y doesn't sound like a practical deal.

My 120Y owners manual clearly shows that a tach was an option & it fits to the left of the speedo. There may be a clock in this position.
The wiring diagram in this owners manual also shows how it all hooks up.
Try finding a 120Y coupe in a wrecker & buy the instrument cluster. From there, you can either swap the whole thing, or fit the tach. This way, you end up with two Datsuns with factory Tach's