Kelsey Hayes Disc Brake Upgrade Question

For whatever it's worth, my 63 Valiant slant six has KH front discs and the original 7.25 rear with 9 inch brakes. Brakes work great. My 67 360 Barracuda, on the other hand, has the same front brakes and an 8.75 rear with ten inch brakes. It used to prematurely lock the rear brakes on a hard stop until I replaced the rear wheel cylinders with this set I got from Ehrenberg: for MOPAR Disc/ Drum Brakes Lockup Proportioning Fix Plymouth Dodge A, B, E-Body | eBay. (I'm thinking these might just be the standard nine-inch brake wheel cylinders, but I don't know that.) Both cars have disc brake dual master cylinders and the later style OEM combination valve (post 8, above).