I agree with everything Erik says, and add a couple of hardearned pointers too.

hardware
Snub all contactor contacts. Diodes on coils. Star grounds as needed. Don't route dirty returns through digital ground.

Software ? Make sure all interrupt vectors have at least a RETI instruction in them to prevent them executing garbage, even if your code disables interrupts.

Surely for Murphy's sake, you should use TWO watchdogs Erik ;-))

Steve