The factory offered 76mm pistons for the 1200 engine from their competition department back in the 70's & I bored my 1200 block to this size back in about '81, but I used standard size A14 pistons. I drove it for so long that I eventually wore the motor out, so I bored it to the first oversize [76.5mm] & used A14 pistons again. Once again it served a full life & was removed from service only due to a burned exhaust valve.

Plans called for one last rebore to the second oversize [77mm] but an A13 block turned up & I will use that instead.

The downside to this is that the A14 pistons use a 19mm wrist pin, while your A12 rods will have a 17.5mm wrist pin hole. You can have the rods bored, as I did, or you can find some A12A rods in the USA to overcome the problem.
A12's are generally quite safe at 76mm, & 'probably' safe at 76.5mm. After that, you're on your own.

My recomendation is the same as the others. Spend the time & money & get yourself an A15 as the additional torque will help things along very nicely. A12's & auto's are best used in the lightest cars.