Jeez, this would be a tough retrofit. The Swim Platform Inc. platforms are nice looking, but they're designed for swimming and playing. They are add-ons and not intrinsically part of the hull. You get a heavy-ish diver on the end with a dive ladder and some even mildly pitching seas, and the momentary load is going to wreak havoc with your installation. One of the premier selling points of extended platforms is for personal watercraft, and there is no mention of sticking one of these back there.
Looking at a Regal 320, you might be able to engineer-in a nice gearing-up station on the reverse slope part of the transom with some Nida-Core or pklywood and epoxy resins. You would not have too much room back there - 4 singles or 2 doubles might be fitable. Extras would have to go in the cockpit. There are other tank mount options besides Roll Control.
I guess you had other considerations for a boat besides diving or you would have skipped the Regal. Good luck rigging that baby up for diving.