The 87-89t front twin piston calipers are a bolt on modification for cars that don't have them, you need the hubs, rotors, calipers, and the appropriate wheel bearings.
Fitting rear vented rotors to an earlier model is not so easy. The rear suspension arms are slightly different between the 84-86 and 87-89. The larger 87-89 vented rear rotors and calipers won't fit on an 84-86 rear control arm. If you want the large 87-89 brakes on your 84-86 you have to swap to the 87-89 rear control arm. Be carefull if you have extremely large wheels since 87-89 suspension is 10mm wider on each corner - your wheels might rub.
You can also use second generation 300zx 4 piston front brakes with the .170” spacer and 87-89 front hubs with z32 rotors. this requires new brake lines and wheels that will clear the 4 piston calipers. Actually, they will fit on fine without the spacers. The spacers simply center the caliper over the rotor. If you skip on the spacers, be sure to replace your pads before they are too worn. If allowed to wear too far the pistons could pop out of the calipers, and this is a bad thing to have happen while you're driving. The brake hoses are available from SMC Products. Stainless steel replacement brake lines are a great upgrade if you can afford them. They give a much firmer pedal feel and less travel. Motorsport Auto also sells them for $120 for a complete set of 6. Skyline GT-R 4 psiton front calipers and rotors will also fit on your 300ZX with 87-89 hubs and .170” spacer between the caliper and strut.
There are 6 of them. If you upgrade to z32 brakes in the front then buy a set of the z32 teflon brake hoses - the fronts will fit right on, and the rear z32 brake hoses will replace the inner rear z31 hoses. Buy a set of z31 hoses and use the rear ones for the rear outer hoses, and you will have the front z31 hoses leftover. Sell them to someone for a few bucks and be happy.