'66 C20 Wheel Cylinder Bolts

[Rust-O-Matic]

'Mornin fellas, new guy here. Been lurking around here for a couple years though. This site is a huge wealth of info, so I'm hoping somebody can point me in the right direction on my latest endeavor.

I'm replacing the rusty, crusty, froze up brakes on my '66 C20 and I managed to mangle the head on one of the wheel cylinder bolts. There is two in each cylinder, that go in from behind the backing plates. They are 5/16" x 18 thread, 1/2" long, and require a 3/8" socket to remove them (or a 9mm custom fit with a hammer after you booger up the head). The head size is important, because it's smaller than a standard 5/16" bolt would be, giving clearance to get a wrench on it. They look pretty much like standard header bolts, but shorter. For whatever reason, I'm having a hell of a time finding replacements. I'd like to just replace all of them if I can find some because they're pretty rusty.

I've tried all the usual places, Google, Advance, Oreilly's, Rockauto, LMC truck, half a dozen local hardware stores, and they either don't have anything similar or the guy behind the counter has no idea what I'm talking about. There's lots of "GM style" wheel cyl. bolts on eBay, but none of them appear to have the smaller head on them like mine. I assume GM put these things on bazillions of vehicles over several decades, so I'm surprised I can't find any. Maybe I'm not calling them by the right name or something? Does anyone know where I can get some of these? Any help would be greatly appreciated.