Always be suspicious of "A14 GX". There was no such thing. In japan there was A14T twin carb engine which had the regular size-port A14 head. And there was B210 GX which came with regular A14S (single carb) engine.

Datsun Competition sold a "GX Cylinder Head (from 7/73)", which was an A12T part. This is very similar to the A12 GX part, with the extra-large oval ports, and small valves, but for the new type engine (1974-up engine). It was never factory fitted to A14s.

They also sold a "Competition Cylinder Head (from 7/73)" which was a polished and ported version of above, fitted with the larger A14 valves.