Use the 65-73 original drum brake cables. Weld a cable stop to the axle u-bolt. The one in the picture was made from a 1/2-13 nut, welded and ground down. Modify the lever on the caliper to fit the cable stop. I accomplished this by using vise-grips to collapse the end of the lever so the stop on the end of the cable would not pull through it.
Caliper originally on Right side of the GT goes on Left side of the classic. It mounts behind the axle instrad of in front as it was on the GT.