I'll leave the analysis to the welder experts, but I would offer this comment.
Make 100% sure that your welding ground is as close to what you are welding as possible and that you are not getting any transient amperage running around throughout the truck frame and electrical system.
Welding currents wandering around looking for the ground will fry the coil, points, regulator and anything else that can be discovered.
But thow can that be? Well, trucks are fraught with rusty connections that divert electrons from intended paths, and welding amperage will walk through rust and look for unintended paths of least resistance.
