The problem is the angle that the foil and halyard knot are at... they are just too close, and the knot is just a wee too big. The foil appears to grab the halyard at the knot...

I will play with this a bit with some other shackles to make sure I can get the stretch I need...

But yes, thanks, it was my thinking that a halyard shackle is a halyard shackle... one simply chooses a shackle that will handle the load. Of course a bit of style also matters... I like retained pin shackles, for obvious reasons.