Have the battery load tested(not with the tester the Autozone uses). I had the same problem with a 82SD and it was the less than 1 y/o Delco battery. It would start fine for weeks then it wouldn't.

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1971 R600 Mack toterhome (SuperDawg)
99 NU-WA Snowbird 34+2 SE 102 5er
If it's not diesel, it's not in my stable!
53 F100 617.951/auto
95 Dodge 3500 Cummins/5 speed service truck