Intermittent electrical sounds like corrosion/rust on the grounds from battery to block and block to body. Check those first to ensure you've got solid grounds everywhere-especially as you mention that the rig had a lot of problems when you got it. It's not outside the realm of possibility that it's been grounding from block to chassis through the drivetrain all these years.

It's also the super cheapest and easiest first fix that could rule out a bunch of other, not so cheap and easy fixes.

1965 C60 school bus | 1967 GMC 6500 school bus