Wheel cylinders for 11 inch drums

I bought a 65-70 B-body 8.75 inch rear to wedge under my 69 barracuda. The rear came out of another A-body car so I do not know the original application. It also came with massive 11 inch finned drums. I have tried to replace the wheel cylinders, but all of the chain auto parts stores have a correct appearance replacement part with undersized attaching bolts. When I place it on the backing plate, it has excessive play because the smaller diameter bolts will wiggle around inside the larger holes. I am afraid that if I install the replacement parts they will eventually begin working loose,and maybe wallering out the bolt holes. I cannot rebuild the original wheel cylinders because of pitting in the bores. I do not know what the attaching bolt size is, but it takes a 1/2' size socket on the bolt head. Bore size is 15/16". Does anybody know the correct application, or a manufacturer and part number, for me to get correct replacement wheel cylinders? THANKS