Just for the info : I fitted one of those "right stuff" kit (ref AFXRD01) on a stock axle with 5-lug arms.
They are cheap, but quality is OK for the price.
Caliper position has to be upper-rear. Fits perfectly on the axle, caliper and pads are standard GM, easy to find. Brakes very strongly (even a bit too much !). Bleeding fluid procedure on this car is a pain, but this is another story.
I had to cut some steel to make them fit. When the suspension collapses, caliper would hit the barrier, behind the wheel arch (which is not very wide). So you have to make some room to ensure the suspension will work properly.
Thankfully, there is some room to cut, just watch for the panhard bar mount, left side.