Assuming you work from a base of 10 shackles, which in all fairness are readily available from the dirge and drudgery of Anne the Stylist...
You will use a minimum of 250,000z and 50 elunium (if you assume the worst and all shackles break at the +5 mark) just to even have a 40% of making one +7 shackles from the remaining 2.4 shackles from the +6 - +7 mark. In the end, you'll end up breaking a load more then 10 unless lady luck happens to be smiling upon you.
All this for 2.1% DEF, on a heavy item that wouldn't be used by any class worth its salt in the long run.
I don't recall there being a necessity to have +7 shackles to activate the attack bonus from the balls of blood. It just says "[+ shackless] + 100 Attack" or something along those lines.