Most FSJs that I have seen have discs already but for the early ones like mine that need some more stopping power there is a cheap answer. This conversion has been covered in the mags and on more than a few boards before but I'll try to sum it up for you who are looking for the upgrade. This is what I used on my Commando and am going to use on my Wag as soon as I can get to the junkyard for the parts. It will work on any early 5 lug Jeep and uses the 6 bolt caliper brackets from a later Wagoneer along with the calipers and rotors for a 77/78 Jeep CJ7. These must be the 1 and 1/8 inch thick rotors to match the stock Wagoneer rotors thickness and are 5 lug.
1) Bolt the caliper brackets to the stock spindle (this should be a bolt on, maybe some grinding on the lower part of the flange where the part number is cast in).
2)Check for clearance of the calipers on the brackets and grind the spindle assemblies as needed to allow for their full range of travel. This will vary depending on if you have a Dana 27 or 30 or whatever.
3)Turn down in a lathe the back of the stock hub to allow the rotor to be mounted to the back side of the hub using disc brake studs (length depends on application).
4)Mount the rotor to the hub and mount assembly on spindle.
5)Mount caliper on bracket and check for clearance over range of motion. Also make sure caliper is centered on hub and pads do not bind against rotor.
6)Mount rubber brake lines; for longer lines use Chevy lines of the same year; about 5 inches longer. Also make sure your calipers and lines match in year as there was a switch to metric banjo bolts @1980 or so.
7)If your master cylinder was set up for drum brakes you may be able to use it by removing the residual pressure valve or swap to a later disc/drum master cylinder.
This is just a rough guide to the steps for this conversion and if you have a later 6 lug FSJ it will mean using later model rotors or swapping the complete axle. For a more detailed article check here:
This uses pretty much the same hardware that us 5 lug guys will need and works great.