Hi,
It all depends on a couple of things. Did your car originally have disc brakes, if not, where did the setup you have now come from.
Besides the main caliper brackets you may need more.
For a 68 dual piston caliper, a special rotor is required.
1) There's the original two piece.
2) Repro (one piece) has more of a bevel on the hub part, allowing caliper clearance.
3) there is also a Repro two piece, I have no experience with that.
You will also need new brake hoses, if 4 piston calipers don't come with them, you will need short little hard lines and brackets that attach them to the calipers. Single piston style calipers have a different hose.
You may need to replace the brake hose frame brackets if they were replaced.
Spindles depend on what you really have on your car.
Hope this helps a little.