is your return spring on your throttle, your choke closed(at least immediately, after some starter and attempts then it should logically not be as closed hopefully) and your throttle at the idle stop(Aka the throttle cable is good, throttle at idle not WOT) and how is your distributor turned, bump it and maybe it was loose? maybe your mechanical advance is kicked out because rust or springs weak in distributor
also is your spark good and strong, take a plug and test spark through a plug like yours and check for fouling too, maybe some fouled plugs and spark is not strong and thus isnt producing the needed oomf in addition to other small things that could add up to your situatio
plug wires good? any carbon tracking on the plugs? any etching or loss of material on the cap towers? Any old dirt and stuff? I would clean plug tips, cap towers with denatured or rubbing alcohol and some shop towel to ensure no conductive buildup is able to provide a path to ground beside the intended tower/wire nubs, ensure your wire is making proper contact and is not high resistance with ohm meter and clean the plug wire ends with alcohol and swabs. if wires are due for change you can skip this step with a new quality wire set. napa belden wires are good value for good wires. if plugs are too fouled or gap is not in spec, you could be fighting uphill on that or maybe shes floooded and could use some carb spray on them and dry out of cylinders. wire order proper? Its amazing how sometimes you can screw it up or be one off in the right order, just started on the wrong towerr on distributor lol. Pull the coil cover, no damage visible and in good shape? evidene of arcing to the plastic at all?
i remember someone who had a ignition switch issue where the coil wasnt getting consistent power in crank but always had it in key on so it would spark and such but the power was turning on and off and probably couldnt build and collapse as needed to send enough power on each cylinder but would still cross a tester etc
No vacuum leaks? Unplug all the emissions all the extra, you should have one into the pcv valve and brake booster but if you really wanted you can plug those temporary but I would just verify the PCV is ok and if you wanted to plug the brake booster line it would just be easier to verify that one large one to be absent of leaks but you could do the old wrap a little tape around bolt threads and use a flange bolt that fits in hose end to plug it to rule out a failure on that fun stuff
If a auto trans, is the PN switch working right, someone may have rigged some wires here or there and ended up with a present where you have a starter turn but a worn switch or misadjusted etc is cutting ignition power but starter power was provided by alternative routes so it would spin no matter what. been there, sorta. still needed to be out of gear on my car but the antitheft system was designed to prevent starter and fuel injectors without a valid key but someone took the PCM out of the equation for the starter but left the fuel as it was so i could use a key cut for my door that I never programmed to spin the starter but it wouldnt start(fine for me, but when your mom runs your battery dead trying to move your car on a cold morning with your door key... or your diagnostics would normally tell you to look elsehwere if the starter cranks but car doesnt start(because it was designed such the antitheft would not ever fail in that way to allow crank but not start with factiory wiring). Then your truck is much simpler than my car and older too, someone may have gotten ambitious with the old household wire nuts and personalized head scratcher for you down the road.
Im bored and gave way many things to verify but hopefully helps
I remember