Misses under load

My 69 440 Six Pack was running great then all of a sudden, it started missing under load. Idle and revving in neutral is fine. As soon as I put it in Drive, it starts to miss. I checked the wires and did a vacuum test. Everything fine there. Suggestions?

Start with the spark plugs. just replace them.. everyone has a favorite.. I like Autolite...

could also be, a bad wire, or the coil could be arching from the primary wire to secondary wire... (replace coil).

It takes more voltage to fire a cylinder under load.