John, You could mount a cheek block on the side
of the mast near the head. Then attach a cleat at the base of the mast just below the boom. Then run your topping lift from the cleat through the cheek block and down to the back of the boom. You could use 3/16 to 1/4" line to make it work. This way would allow you to adjust the length of the topping lift.r.w.landau