I have a 82 dodge d 150 with a 318 my truck was dying and was not charging so I changed the alternator and it worked until I turned on the headlights then it died I noticed that the belt was loose so I tried to tighten it and then it ran fine all day then that night I turned on the lights and it started dying. Now the truck won't stay running it stars but then dies unless I hold the key down what could it be

Replace the ignition switch.

If your truck only runs while you hold the key is in the start position, the ballast resistor is shot. Small white thing about 3' long with 1 or 2 wires at each end mounted on the firewall. When you start the engine, the coil gets 12 volt through the starting wiring (for faster and cold weather starts) and with the key in the run position, the ballast resistor supplies about 6 to 8 volts to the coil. Makes the coil and other parts of the ignition system last longer. One way to test is pull the wires off each end and hook them together with a paper clip or piece of wire and start the truck, if it stays running, there is your problem.
Good luck.

Double check that your battery is good. Follow your battery cables and check for any torn areas. Take your alternator to get checked, could be a bad alternator to begin with.