8 3/4 wheel cylinders

Ok, I am wanting to rplace my wheel cylinders. All i can find at the local car parts places are what appears to be the right size diameter cylinders but the mounting bolts are a smaller diameter than what i have. i do not want to take a chance of these not being properly secured. The bolt holes on the backing plates arnt holes at all but slotted so the right diameted bolt seems to be important.

Any idea where I can find relacement cylinders? P/N would be very helpful. Thankd FABO