Compete USA made rear wheel cylinder assembly with bleeder (picture for illustration purposes only).

Note that the difference of rear wheel cylinders from (not including...) 1976 is they enlarged the bore to 15/16th  .. FROM . .this one.. 7/8" (22.2mm).

You can also get a better deal with the brake shoes and cylinder kit for a discounted kit..which is listed separately too. 

Two required per car.. or individually they are the same units.... and if your doing this job properly.. the normal patern is one will leak.. you fix it. then the other will leak being the next oldest weakest hydraulic part.. so it's a good idea to do both at the same time... and they are certainly cheap enough now from usgmsp.com.