OK - backfire. Backfires are caused when the spark goes off while the intake valve is still open causing the fuel air mixture to ignite in the intake manifold & blow out the carb.
There are several things that can cause this. I'd make a list & start from the top.
Ignition - ensure points are good (sounds OK as the truck runs)
adjust the timing. If your timing is off you can have all sorts of problems.
Once that is good adjust the valves as mentioned above. These old trucks need this on a regular basis unlike a modern car that adjusts these automatically you must do this yourself. A valve that is out of adjustment may not close in time for the pop.
If you are still having the issue after that look deeper. Since you state the truck runs well when cold you may have carbon build up inside the cylinder. A carbon hot spot can pre-ignite the gas causing this as well.
Check the compression. If you have a valve stuck or out of adjustment this will tell you which one.
Process of elimination. Hope this helps you narrow it down.
