Although i have never done it, i have no reason to doubt that the later 1200 blocks will bore to 76mm. Nissan once supplied 76.8mm forged pistons for the A12 motors.

To prevent the A14 crank from touching the bottom of the A15 pistons it is necessary to chuck it in a suitable lathe & machine the dags from it & reduce its overall rotating diameter a little. This basicly brought all the counterweights to almost the same diameter on the crank that was used in the local engine. [All counterweights are not necessarily the same on any given crank] This is a bit of a trial & error job, but the amount needed to be removed is small. There is no "grinding to be done".

Some rods have a lump of material on one side with an oil squirt hole to lube the bore. Others have only a small bump with this hole. During trial assembly, make sure that there is no "lump to bore" interference. If there is, then get a nice sharp round file & remove some material from the offending area. Only a small amount will do the trick. I suspect that most rods will not have a problem at all, but check the clearance & make sure that there is about 1mm or more. It is only the very bottom of the bore that may have a problem, & only on one side.

After you are happy with the crank, you can send it out for grinding of the journals if needed & for chrisake, make sure you get the whole rotating / reciprocating mass ballanced.

Use an A14 or A15 cylinder head as the port, combustion chamber, & valve sizes were designed for this capacity. If doing it with an early A12 block, MAKE SURE that you modify the head to the early oiling system. If using a late block, it just bolts on.

It's gotta be a good thing to take the original 1172cc out to 1434cc.[with 77mm bore] Thats 262cc or about 23% greater than stock.

Used A15 pistons will work just as well provided that they are within service limits in relation to piston skirt taper & ovality, the ring lands are unworn & undamaged, & the pin bore is within limits.
Usually the pistons are fine, it's the bores that wear. If using "pre loved"pistons, then the reconditioner will bore & hone to suit, but you will need to ensure that the ring gap is right as the new bore may be a poofteenth smaller than with new pistons & the ring gap may be a tad tight. Just check everything as you go & you will love it, especially when your mates become amazed just how strongly that dinky little 1200 motor of yours runs.