No spark I Install new distributor?

79 could have a 302, 351, 400 or 460. There are 2 possible firing orders depending on which engine you have if it's the original engine. Set it on TDC cylinder one on the compression stroke. Then pull the cap off and look at where it's pointing. If it's at a terminal post, make that #1 and wire around counter clockwise following the correct order. Put a spark tester on the coil wire and see if it has spark out of the coil. If you have spark out of the coil and it's wired correctly then you should be able to put a timing light on the cylinder one wire and set the base timing.

OK, no spark from the coil can mean coil or the trigger is bad or more common the ESC box is bad. To test the coil is simple. Take a piece of wire, connect it to a good ground, connect the spark tester to the coil wire, turn on the key, now tap the grounded wire on the negative coil post, do you get a nice hot spark Yes= Coils Good, No= replace the coil. See if you have spark. Still no spark = find the box shown, Find the part number on it, get a replacement. As for engine size, if it's stock there should be a sticker on the radiator support that tells you the engine size, timing and spark plug gap.

Just a simple gap style tester, you could even use a plug wire and a plug. You are just testing if the coil fires on it's own.

That sounds like the coil is OK but the ignition switch has a problem. How long has this truck sat around? It could be corrosion that some contact cleaner would help with, however being a 79 it may just need a new switch.

You pretty much already tested it. If it stopped cranking with the key then it likely isn't turning on the power to the ignition either. They are all related. The switch controls power to all the other areas and if it's not spinning the starter and now the power to the coil is out unless you jiggle the key, that sounds like a bad switch.

Exactly what is it doing that is the same? No crank and no spark? You replaced the electronic switch behind the dash correct? If so you still have either corrosion or damage to hunt down. The service info I have doesn't have the wiring diagram for that vintage but I did find https://www.fordification.net/tech/wiring.htm and they show it as images at the bottom of the page. If you go there and download those so you have a reference we can do some testing.

OK go grab those images and I'll look at them to see where things go, have to dust a few cobwebs out of the mind as the only vehicles that age around here are collectors and might be in a shop once a year. But I'm sure we will figure it out.

You test resistance from the + to - (primary winding) and then from the coil tap to each terminal (secondary winding). So first set your meter to the lowest ohms setting, now short the leads and write down the number that shows up (if your meter has a zeroing feature, set it to zero. Now put the meters plus lead on the coils plus terminal and the negative on the negative. You should see between 1point 3 and 1 point 8 ohms. Open circuit or higher than 3 ohms and it's likely junk. Now switch the meter to 20K ohms. Then test between the positive post and the coil post. There you should see between 8,000 and 12,000 on a good coil. Then check between the negative post and the coil tap and it should be the same reading within an ohm or two.

So a good coil might be 1.6 Primary and 9,480 ohms Secondary.
If you look on the coil you have and look up the part number it will tell you what the specs are but the numbers I gave are common numbers for a good coil.

Does your meter go lower than 2K The low reading is basically one point three ohms to one point nine ohms..002 could be fine because the meter rounded up. What would be ideal is 100 ohms if it has it or try diode test if it has that.

There isn't much to test, does it have power and a good ground. Beyond that they either work or don't.
This video covers a lot of the issues with the duraspark system. There are multiple others as well. https://www.youtube.com/watch?v=RylhE4_HSPU Might be easier than following written words, especially coupled with the wiring diagrams.
Also you mentioned that turning the key does nothing, do you have power to the gauges and radio when you turn it to on? Then it does nothing when you turn it to start correct? Or do you get nothing? You said it was running ok a year ago but now doesn't so something changed. The common things are corrosion from setting, go over the battery cables and every plug and connection you can find and clean them especially grounds. Then go to the coil, do you have 6 or12 volts at the coil with the key on? How about when in start? If not then the problem isn't in the ignition system but in the wiring of the truck. What items work in the vehicle? Wipers? Lights? Radio? Gauges? Go to the fuse box with a simple test light and check all of the fuses with the light with the key off then on.