There is actually nothing wrong with drum brakes on the rear, particularly in street cars, & there are many who would tell you that they even have some advantages over discs for rear axle applications.

If you are running stock type Datsun 1200 discs on the front, & they were not factory fitted, then make sure that the rear wheel cylinders are the correct, smaller ones.
The discs need a higher line pressure to provide a given level of retardation. This higher pressure tends to make the rears lock up a bit too soon, so they reduced the rear wheel cylinder diameter to help balance out the braking force, front to rear.
If running the larger bore sized cylinders, the mismatch could be most noticeable in an empty ute on a wet road whith a hard brake application. Oops.

If your ute has the larger H165 diff [& it should have], then the diff from a Bluebird will fit right into your original rear axle housing. The original ute ratio was 4.1 to 1 & this is not much fun in most street cars, so the 3.9 to 1 ratio from series 1 & 2 sedans & wagons that came with a 5 speed or an Auto will help. Another option is the 3.7 to 1 diff ratio from Bluebird sedans that came with a 4 speed.
Forget the wagons that came with the 4 speed as they used a diferent diff