To answer your original post, YES- electric welding of any kind for any length or time on a car is very bad for any connected electronic devices. This definitely includes MSDs as well as Ford TFI boxes and anything similar. While electronic ignitions and EFI controllers usually fail entirely, occasionally the damage is such that they continue to sort-of work for a while. I would at the very least send the box back to MSD and ask that it be tested.
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