Lock cylinder replacement?

The ignition switch is a separate part from the lock cylinder. Your truck should be one of the last of the dash mounted switches, Ford changed from dash to column in 1980 on most models. As such the switch for yours should be the part shown in the first image. It is a housing that the lock cylinder slides into and turns the internal switch at the back. The lock cylinder is the section that turns with the key in the second image, all it does is rotate the switch. The detents and contacts are in the housing. So if the issue is that the key doesn't work in the cylinder or the key falls out regardless of where it is turned to, you replace the cylinder. If the issue is that it turns fine but the starter doesn't work or the switch doesn't have defined acc-on-run-start positions or that you need to wiggle the key around to get the switch to work, you replace the ignition housing in the first image.
It's been a while since I changed one but it should be simple, First disconnect the negative battery terminal. Now go inside the truck with a straightened paper clip. Turn the key to the accessory position. Now insert the paper clip in the small hole in the cylinder and push in, now rotate the key counter clockwise and it should go past accessory and unlock the cylinder from the switch housing. Then you pull the cylinder out using the key as a handle. Now reach up behind the dash and unplug the large connector on the back of the switch. Now there is a special tool to remove the retaining ring that you see on the front side of the dash, it is a nut that screws onto the cylinder. I've used the tool, as well as a padded pair of pliers and even just a flat screwdriver to rotate it. You can sometimes hold the ring in place and unscrew the switch itself from the ring. With it out you simply reverse the process to install the new one. Normally the new switch will come in the correct position for the lock cylinder to be installed, if not look in the old switch and use a screwdriver to position the actuator opening in the switch to the correct position. Then you push the ignition cylinder in until it seats and remove the paper clip, The clip should engage and the cylinder should latch into place. Test that you have accessory - off - run - start positions. Then plug it in and reconnect the battery and verify it all works. Now IF the lock cylinder itself is bad you just do the first few steps, but that is usually not the case for things like no start or electrical issues as the cylinder doesn't really do anything other than rotate the actual switch.
Plus if you have a working cylinder that key should fit the other locks, if you replace the ignition cylinder you will need two keys, one to deal with the ignition and the other for the other locks. This isn't a big deal until you accidently try to use the wrong key. To prevent that I usually file a couple notches in the head of the ignition key or similar so that you can tell it apart before use.
OH - Found this that might help, it's a Bronco but should be the same. The 79 you have should use the same style. https://www.youtube.com/watch?v=Qconvy82mwk

If it is the newer column mount the switch is still a different part but now it bolts to the column under the dash, with a rod that activates it from the cylinder. Those are pretty much the same in how they work, the cylinder just moves the switch, so if the issue is electrical it is the switch, if it's mechanical you work with the cylinder or the housing in the column. Let us know what you find.