superfatto's kits just work -- good stuff
If you prefer to do work on your own, here is some more information
The South Africa 1200/1400 bakkies -- starting 1976 -- use the same struts as 1976-1980 B210. They have different hubs which use smaller bearings to fit a larger stub axle. 1973-1975 used larger bearings and hubs to match. In other words, Datsun upgraded the struts for 1976 (technically DEC 1975) to use stronger/larger stub axles
I would guess 258mm brakes are large enough to use with RX-7 FC 4-piston calipers. The FC model means 1985-1992, the 4-piston was 1985-1988 and only on 5-lug wheel models. Lesser FC versions used a single-piston caliper. The 4-piston was used with a 276mm disc rotor, and most Datsun guys have used 280mm rotors or larger with the 4-piston caliper
That B310/120Y caliper bolt spacing may be referring to Australia 120Y which used Australia brakes and a caliper adapter plate. In any case it matters not, use an adapter plate to mount the RX-7 caliper (for the Datsun models where the Hilux bolts directly to the strut, the disc rotors are machined down to fit the caliper, e.g. it only works with certain size rotor). By using an adapter plate instead (a piece of steel with four holes in it) you can make a caliper work with a variety of rotor sizes
RX-7 four-piston caliper used with Datsun strut
The RX-7 four-piston caliper won't necessarily stop your car faster than a single-piston caliper -- but will likely have better pedal control and different pedal effort. And if the brake pads are larger that will stop the car faster. And compared to many calipers there are more choices of brake pads for the RX-7 (various racing type pads, street type pads, etc)