According to this site, Mainsheet Load Calculator
20 kts apparent translates into 1069 lbs. Divide that by 5 and you get 214 on the tail. Can you pull over 200 lbs? I know I can't, that's why you have a winch. Consider adding a fine tune adjustment and doubling your purchase.
Also, you can try the smaller blocks, but if they grenade and take someone's head off, you can't sue harken. Also, the shock load on a jibe will be WAY higher than your static sailing around.