MY kinda truck!!!
OK,, it goes like this. Sounds like you are into the rear brakes at this point. That splined part is the rear axle shaft. Smack it with a hammer and it will jump out at you. HIT it, don't make love to it! Pull the shaft out and you will find the spindle nuts. Seems like they are round with slots for a spanner wrench. (I was into my 71 5500 last fall, memory may not be perfect on all this.) Pull the hub and drum together, watch your toes, it's heavy! The drum is hooked to the hub and the wheel studs come through both pieces. Assuming you have disc wheels there will be 10 wheels studs, not that you have to mess with any of them to get it apart.
Anyhow, I made a spanner to remove the spindle nuts, they are available commercially but I'm cheap and handy. Get the spindle nuts off, then rock the hub to pop the outer bearing loose, catch it when it falls out, no point getting it dirty.
If you had a fire then nothing to worry about hurting the seal. Slide the drum/hub off the axle tube and you're there!
Brakes are probably Wagner, self adjusting. New linings can be put on your old backer plates, a truck shop does that regularly. TWO wheel cylinders per assembly, if you burned you'll need to do both of them. Only one bleed screw, on the top cylinder. Better plan on doing some lines as the originals are probably rusted into the wheel cylinders. Spring kits and hardware should be readily available, I didn't have any trouble getting parts for mine.
Good luck, even if things look good, if it got hot it's best to consider it trash and replace it. These bigger ones are no fun to tear into so it's lots better to do it right once than to go back in again
Keep us posted as to how it goes, please.