I assume your 12-bolt is not from an A-Body - if it was the mounts are already cast in to the housing for the triag 4-link. Here is my '56 truck frame - I used a 70-72 Olds 10-bolt 8.5" rearend (no c-clips) and bought the coilover mounts from Ride-tech (they bolt to the factory lca bracket. Removed the coils spring mounting pads to clean things up a little. Used johnny joints on all links. Of course, you'll have to make frame mounts (I don't know if anyone makes them for the TriFive trucks). I used S-10 disc brakes. I did c-notch the frame.