As Tally Ho says, my main halyard and topping lift are both 3/8". My end attachment is the same as Tally Ho - a small shackle. The factory rig is to take the topping lift from the end of the boom to the top of the mast, down the inside of the mast, exit 5' off the deck, and to a mast cleat, which is how I used it for 10 years. Recently , I have run the topping lift back to a port cabin top winch I recently installed. I do have to adjust the topping lift slightly, because I want to lift it up a few inches above my bimini when tied up, but release it those same few inches underway, so that close hauled the main has a nice tight closed shape (no twist) in light to moderate winds. If I did not have a bimini, it could stay the same place all season. I would not want less than 3/8" topping lift, unless it was dyneema.