My 315 poly keeps running after everything turned off..

I was thinking; that the 58 has an old-style mechanical regulator with point-type switches. I forget how exactly they work, but it sounds to me like yours is sticking in the run position, or is not being properly controlled by the ignition switch, or is just wired wrong.
If you have a working ammeter, it should indicate which way the power is flowing, and it should show straight up when you shut the key off, indicating no current flow.
I think.....