The bigger-braked B210 & B310 had 13/16 rear cylinders. The Datsun 1200 disc-brake model has 11/16 ... so when upping the brakes this sugggests that you should fit larger rear cylinders.
The good news is that drum-brake 1200s already have the 13/16 rears, so there's your source.
I was told this past weekend that P510 rear wheel cylinders are 13/16 and bolt right into the 1200 rear. So there's another source.