If you’re going to race your boat don’t use a shackle. Due to the excessive mast rake carried by the 16 when racing you want to limit the distance between the boom and the back beam. That means low profile blocks and no room for big shackles. Attach the harken block directly to the traveler car with the 3/16 pin. You will need to bend open the traveler car a little bit. That’s how the fast guys do it. I’ve had mine rigged up like this for a few years and never bent the pin.