I was advised to buy a Freedom Plate back when I got into DH diving and I chose not to. Really wish I'd done so now.
Having said that however, I've dove DH regs with a variety of harnesses and BC's now and I've found a few things that work. The number one problem with modern BC's and BPW's is that they use two cam bands and the top one is too high to allow for proper tank mounting with a DH. My BPW is my best modern rig, yet my worst for diving with a double hose. If I combine that with an 8" tank it makes for an almost unbreathable double hose experience. The tank ends up too high and the reg sits way too far off my back. A modern backplate with a STA sets your tank about 3/4" away from your back to start with, then the E-8 tank adds another 1/2". My best DH setup is an old set of Sportsways doubles with the original plate and harness. Those things couldn't sit any closer to my back and they're secure and comfortable to boot. With the manifold set up with the outlet facing down you can wear the tanks higher while still keeping the regulator low.
I've also got an old plastic backplate from the 60's. It's got a psychedelic swirled pattern and looks like something Jerry Garcia would have used. That thing is also great. It very much resembles ZKY's freedom plate except it is made of plastic and sets the tank about 1" away from my lower back. This is good for clearing a weight belt. At the topside though it sits flush with my shoulder blades.
I've got one of those old hollow plastic plates as well. They're very easy to find, (just take apart most old BC's), and they work pretty well with DH regs. They do add a little more distance between your back and the reg though. Not a lot, so it's not a major problem, but there are better plates out there. Still, you may have one floating around and not even realize it, and they do work reasonably well. You can also fill them up with lead shot to add weight to them.
As I mentioned earlier, tanks make a difference. LP72's were 6.9" in diameter, modern tanks are 7.25" or 8". That sets the reg further from your back, especially with the 8" tank. Also, the LP tanks breathe better with the unbalanced DH regs. 3500 psi air and olld unbalanced regs just don't mix that well. That shouldn't be an issue with your Phoenix, but you'll find that DH regs are like potato chips and you can't have just one. Of course, you can always let some air out.