Yeah good luck with trying to get the kits at your local parts store, even with their part number.
"Datsun? What's that?" Been there and got the same deer in the headlights look. But here are a few options.
Front - go with the new wheel cylinders for the 520 from your local parts store. All fittings are SAE and cost is around ~$20.00. This WC is the 13/16, but will work. This new kit will require a little work as the square piece on the wheel cylinder body will need to be ground/filed down a little. For some reason this stub catches on the drum. No idea why, as the 520 and 320 have all the same drum brakes parts.
I'm going with this option for Whitey. So upgrading to big brake! A whole 1/16in bigger
Next best option is to look on eBay for front kits. There have been some for sale, but hard to come by. I snag'em when I can. Goen, Beck-Arnley, or others. Just type in some key words. Datsun Brakes, Datsun Front Brakes. etc. These key word pull up all models of Datsun brakes.
Rear - same as the front. Buy the complete new WC for the 520. Fittings are SAE. Rear WC are more common on eBay and about the same as the parts store price.
Another option is to buy the least expensive 3/4 inch WC, Brake MC kit you can find for a Datsun and just use the new seals. With this option you will need to re-use your WC boot/covers.
Give me a few and I'll post some part numbers, pics, and some links to parts.