Keep a stong twist on with your grabbing hand and also try to push his hand into his arm pit forming a chicken wing. Your free hand should push his elbow straight through the center of his head. The more off balance uke is from the start, the more outstretched his arm can be.
It's important to remember that while practicing a given technique, uke must go along with it. If he resists, he is creating the need for you to switch to a different technique in order to use his energy.
From a shomenuchi, block/grab and continue pulling slightly to draw uke off balance, then in a continuous flow, circularly reverse the direction and cut across his head.