Need New Drums

I am, yes, but I don't know what year the axle is. Since we've narrowed it down, how do I tell the difference?

Also, if I wanna replace the hardware inside, I guess I'll need to know which to get. Everything looks okay, but for safety's sake, new cylinders are probably I order.

Yes, you're correct that the bolt pattern is 5 x 4.5. I have one of those wheel disk thingies that you put over the studs.

If you follow the link in post #8 it shows how to measure the backing plate offset to determine which size brakes the backing plates are for.

But since you’ve got a B body rear axle housing and BBP axles my bet would be you have the 10x2.5” brakes, since B-bodies never had SBP axles or the 10x1.75” brakes.