Joe doesn't even use Facebook, it would have to snow in hell before he wrote a blog haha!
On the English Warbow Society forum, he did write that he uses a single 7" Z-splice, for bows over 100# that bend in the handle. He also mentioned as above that the splice is stronger than the rest of the bow, so it's "easy" if you know what you're doing!
By the way, the arrow you've described (tapered poplar, 7.5" fletchings) is not a Standard arrow, it would fall into the category of a Livery arrow. The Livery arrow is a specification owned and designed by the English Warbow Society as the closest attainable replica of one found on the Mary Rose. It's very different to the Westminster arrow which is the only extant medieval arrow. The Standard arrow - if you were to use the BL-BS/EWBS specification created by Hugh Soar - would be 3/8" parallel shaft, 6" fletchings and a type 16 head or bodkin at a push.
However, if your arrow is 54 grams, that IS the right weight for a Standard arrow. The Livery arrow should come out at around 63 grams. I'm surprised you managed to get tapered poplar to 54 grams! That's super light even for poplar.
Anyway, that's an aside - the bow looks great! If you're getting handshock it generally means one of the tips is coming back faster than the other, resulting in an uneven snap to brace height. A few tweaks to sort out the timing of the limbs and you should be good.