A stock 440 w/ standard bolt ons and tuning will run 12's. A friend had a 70 GTX w/ a 440. (It was blueprinted to stock specs.) It ran low 13's w/o trying too hard. (Closed exhaust.) It had a 750 vac sec on a stock intake, .484 purple cam and headers, stock torque converter and 4.10 gears. It was a lot heavier than an a-body.
I would make sure your chassis is strong enough if you use a 512!