Do NOT use a stock 440 plug UNLESS you are using stock 440 heads. Those stock heads use a J series Champion and most aftermarket heads use the N/C series Champion and they ain't the same.
If it's a stock head, you can start with a J12YC and tune it up from there.
If it's aftermarket heads, you can start with a C12YC and tune it up from there.
I use the C series because the 5/8 hex gives more room than the 13/16 hex.