If you are truly looking for the cheapest bang for the buck. Then you need to find a '73 to '76 A-body with disc brakes and do the swap.
1.You need to get both spindles along with the rest of the brake setup (you will need some of the small parts to swap over)
2. Purchase new rotors, bearings, calipers, and brake lines from a local parts store for the A-body you got them from. These parts are common and should be cheap.
3. Put the spindles on your car. You can use your stock control arms and stock ball joints.
4. Assemble the rest of the brake parts like you would on the A-body.
You may also want to get the A-body master cylinder, but you can test the braking setup with the stock unit to make sure it works.
This is the exact swap I did on my '71 Challenger and I have not had any problems. I also do not have any brake fade like I did with the drums.
The only problem I came accross with this swap was trying to find two calipers with the same size piston. Most of the rebuilt calipers that you can get at local part stores are not the same. We got lucky and found a matching set at the third store we went to.
I hope this can help you.