What would cause a Chevy 305 to idle in park but jerk and die when you put it in gear?

There are a couple possibilities. First, the lockup torque converter could be stuck. It could be as simple as a faulty relay or it could be the torque converter itself. Second, it could be that the engine is not producing enough power to keep running when it's put into gear. For example, it could be idling fast because of a vacuum leak, but not really producing much power. When the transmission is engaged it could put enough power demands on the engine that it can't keep going.