Mean Coyote
Your motor is more than big enough. You could technically run a home generator with that puppy!
Someone mentioned that steady amperage is what is necessary, and that is absolutely true. Also, like they said, hard to accomplish. Your voltage and amperage will change with load (before/after arc initiation), and also with engine speed. Ideally, a movable core isolation transformer would be useful here for this. However, you might as well buy a stick welder for the cost of one which will handle those amps. (or get one from a stick welder.
)
I would get a couple of high amperage 400amp bridge rectifiers, and get rid of the factory diode pack. DC arcs tend to give a better weld than A.C. anyway.
If you want to try your hand at the electronics aspect of amperage regulation, I can attempt to either find or create a schematic if you desire.
Paul Andrulis