Small critique: add (or increase) fillets in you pocketing, what you have it is creating all sorts of stress risers. I am assuming the plates are to be cut on a water-jet, but this would give you the option to do it on a CNC router w/o altering your design. 1/8" or 3/16 should be fine w/o adding too much weight. You also have some weird pocketing at one spot, not sure if that is intentional or not. I can’t see it really causing much of an issue, it’s just not along the line of action of the web.
You may be able to use counter-sunk fasteners on the first-reduction side of the 775 Pro Gearbox Plate, snug the bull gear up to the plate, and reduce the length of the input pinions to reduce o/all thickness & cut a little weight.