Every manual describes preventative measures for arc welding.
I have been arcwelding on cars for near 40 years and have never seen a problem. One would really have to be stupid to cause a problem on that old of car with welding.
ALL OF THE CURRENT goes through the metal to ground. Unless you hooked the ground cable to the alternator those electrons have no reason to take their excited state there.
Like the the other day, I'll have to point out that everything that happens to your car from now on will happen "after the welding".
I'm not good with jokes, but one of my favorites has to do with the farmer who complained to his technician that every since he put an alternator on his truck the lights in the barn won't work.
Funny thing is I read it here everyday.
Bosch Master, ASE Master, L1
33 years MB technician