As has already been said, you should be fine with the steel plate, as long as the bolts are long enough. One of the few downsides of DSS gear is that the bend is shallow, which can make it difficult to fit the steel plate to tanks with short bolts. The Kydex plate has a little "give" in it, so you can make it work with more tanks. (I cannot use my DSS gear on ANY of EE's tanks, because the bolts are too short.)
If these are the old, 2200 psi 72s, be aware that the rig will be very head-heavy. Although, diving where you are, you should need enough additional weight to give you a fair bit of flexibility on where to put it, I would advise not setting up your kit so that you can't put a tail weight on. We use a five to 7 pound tail weight to get the tanks somewhere near balanced.
I started with 72s, and now I won't dive them at all unless everything else is empty or full of trimix. As Bob Sherwood said, at the end of more than an hour spent trying to adjust harness and move weights, "You really CAN'T trim out those tanks!"
