Fast68 advice is well intended; however, itís not entirely correct.
Disc brakes werenít federally mandated on pickup trucks until 72. In addition, there were some spindle, upper/lower A-frames and chassis changes to the front end. The proper conversion is to find a donor 72 to 87 truck and swap the K-frame and the upper/lower A-frame with the spindles and the disc brakes calipers. Swapping these parts is fairly easy as they will just bolt right up to your chassis without any modification and cheaper than purchasing any aftermarket kits available.
Additionally, grab not only the master cylinder but the power booster too. Disc brakes donít work too well without the booster.
Now to address anther issue youíre going to run across. From 72 to 87 Chevy only produced two blot patterns on their pickups 5 and 8 bolts. Your truck came with a 6-bolt pattern. However, there is a simple solution.
General Motors decided to reintroduce the six-bolt pattern for their 87 models and simply took the 5-bolt rotor casting and drilled it for the 6-bolt. Therefore, if you want to maintain your 6-bolt pattern on the front all you need are 87 and up 6-bolt rotors, which are readily available from any jobber. No modifications required.
Now you have it in a nutshell.