Quinn, check ignition fuse, all clean all connections including grounds and check wires for corrosion,tight, undamaged. Check and test battery, safety neutral switch, starter and solenoid for proper operation. Many parts suppliers offer bench test for these components. Check the attached links,instruction and guides, Good luck
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https://www.youtube.com/watch?v=uGGTaOQkkAk
http://easyautodiagnostics.com/misc-index/starter-motor-bench-test-1
http://www.ford-trucks.com/forums/171979-1978-f250-wont-start.html