Madlabs,
Depends on the Welder:
Welder with electronics, may shutdown on lower input voltage or go nuts. Use Variac on output.
Transformer only, input - lower currents.
At work shop has a DC welder, internally has an AC motor / DC generator...
Many types of welders.
Variac works with AC, if welder has DC output cant use Variac there!-)
Welders are time/current rated. Pushing lots of amps do not overheat welder.
BTW: most welders have a lossy transformer. Output voltage is high to help strike an arc, drops while welding. Source of transformer heating. Long term not a good battery charger. Toping off a battery once a while a cheap (not efficient) backup. Of course check out your unit, YMMV.
For my 8 golf cart batteries, bought a fork lift charger for $75, pushes 380amps @12v
Interesting idea to use a welder! A more useful toy.
Have fun,
Scott.