Go with the A12. Those Pommy things are from the dark ages.
There was an A13, but they are a bit thin on the ground. If you bore an A12 from 73mm to 76mm & use A14 pistons, you will also need to bore the rod for the slightly larger wrist pin. It comes out very close to 1300. The block will bore this far with ease & Nissan once supplied 76.8mm forged pistons for racing for use in the A12
My own early A12 engine is currently 76.5mm & has done a full service life at that size. I will be taking it out to 77mm soon & there is one of these A12's at this bore size in a 1200 coupe currently terrorising Hyundi XL's around here.
If you want to do a sneaky 1434cc conversion in an A12 block, contact me. We have already built one with ALL stock Datsun parts. Mine will be next & it's going in a 1000 coupe.