My 1979 g 20 series van won't start. I had the battery tested and alternator tested they both checked out fine. Checked all the fuses they were fine . What else could it be.

Remove and fully charge the battery. Twice this past week I saw where a brand new battery would not start a car. In both cases, charging the new battery for 4 hours, on a 6 amp charger, solved the problem. I would recommend that you do the same and have the battery load tested afterwards. Examine the battery cables for corrosion and hidden cracks. Clean the battery terminals, cable connections, and starter motor terminals and cable connections. Ditto for cleaning all grounds. Next, perform an Ignition By-pass Test, where you attach a remote starter switch or run a jumper cable between the "S" terminal (starter switch) on the starter solenoid and the positive battery connection. If the starter motor now works, the problem is with the ignition switch, neutral starter switch, fuse, fusible link, starter relay, or wiring.

Be sure you tested all under hood and under dash fuses with a test light or multimeter. Visual inspection is not good enough.

I can only speak from my own experience here but in similar circumstances quite a few years ago, the problem was with a fusible link.