Can anyone tell me if a 350 holley is too big for my A12

Using the formula CFM = RPM x CID/ 3456 for an engines consumption of air at a theoretical 100% volumetric efficiency, the A12 engine would need to spin at 16,750 rpm to consume 349cfm of air.

Now, if your round port A12 is capable of reving to almost 17,000 rpm then you might be OK, but at a more component friendly rpm limit of,... say 6,500 rpm, the engine consumes a theoretical 135 cfm at 100% volumetric efficiency.
Very few N.A. engines ever see 100% volumetric efficiency at any point in their normal operating range, so the actual airflow would be even less.

The 350 Holley is probably better suited to a larger, six or eight cylinder engine.