I don't think you want to saturate the core of the transformer. If you did, your inductive reactance will drop to almost nothing. Now have a very heavy iron core resistor.
If the automotive alternator uses the casing as the negative return path, you might be able to 'float' the alternator with a 30 volt zener to get the output up to 42 volts.
Twenty years ago when I used to fix welding equipment, there were some cheap motorized welders that were nothing more than a B&S engine driving an alternator with rheostat controlled field excitation.
Your alternator guy might be able to find a 36 volt regulator used for old boats.
