I didnt use amsteel. It didnt make sense to me as the hook up top was taking all the pressure under way. I used just simple low stretch - and I think it was 3/16. Are you having any other issues other than needing a new halyard in raising the main?
double check the sheave up on the mast head to make sure its spins freely and its edges are in good shape. Since your not pulling that much weight or having it hold tension. Id put the pennies else where on the boat. Just my 2 cents! My low stretch worked flawlessly.