Few, if any, of us will use a harness system like the original, with a backplate and the bracket at the bottom. My understanding is that the original pack was heavy enough that the support bar at the bottom was necessary to hold up most of the weight.
Our fiberglass backed resin packs are much lighter. So, we don't really need as elaborate a set up. I use one of Fettdad's harnesses with the straps from the shoulders hooked into my jet pack hooks.
Then, my divers belt and buckle are attached directly to the bottom of the jet pack with a #8 bolt and nut. The jet pack goes on the hooks and then I fasten the belt around my waist to hold it securely against the backplate. No need for the bottom hooks, and I find that it only pulls slightly on my back and collar armor.