The backstay on my B36 divides to a 6mm wire bridle leading down to 7.3mm "U" bolts through the transom. Don't know why 7.3 mm was used unless that is the metric measurement of an imperial fitting. The wire is connected to the "U" bolts with 6mm shackles, and the same size shackles are used on port side backstay block and tackle. Note, these fittings were on the boat when I bought it in 2010, and have not been changed.
Measuring the tension on the 6mm bridle wire with my Loos tension gauge gave a reading on each leg at point number 8 on the gauge. Scale 8 is well below the minimum shown for a 6mm wire where the scale for that size starts at point 18. Point 18 if reached shows a tension equivalent to 3% of the breaking strain for the wire, or around 100kg, so point 8 is well below 100 kg. because the wire forms a bight, the tension on the single wire above the bight and leading to the mast head will be more or less double that reading, but it is also a thicker wire, and is well out of reach for anyone at deck level to be able to measure the tension on it.
So maybe I don't have sufficient tension on my back stay, but I'm thinking your 8mm shackles would be well able to take care of the job.