Apart from the tachometer that was later added to the A14 setup, is there any other major differences in the harnesses ??. The idea is to remove the old datsun harness and replace with a newer nissan A14 type harness.

if the "older" a12 harness is early 70's it will have the alternator on the other side of the motor, and dizzy in the forward position.

if they both are to suit later engines then i would think they would be very similar, perhaps one has elec cut off on carby and one does not?

Yes, early A12 and late A12 differ mainly by location of alternator wiring. Newer harnesses have some extra wires.

A12 and A14 harness are the same for same year and model, such as 1980 B310

1200 and B310 Sunny harnesses are considerably different even from the same year.

Thank you Guys !!