That isotop bridge looks ok for light use maybe, yes just use 2 of the i/p terminals for 1 phase, dont forget doing this reduces the capacity by the square root of 3, or something like that.
Stick welders are constant current, but you can still achieve reasonable constant voltage with them, they usually have a magnetic shunt to reduce the o/p current, if you turn the amps all the way up and then use a variable inductor to regulate the current you can mig with them.
Have you thought about using a saturable core inductor to regulate the o/p?, then you can use a dc signal to control the current, I messed with this briefly, there was someone on the net talking about it, they had weld sets where I used to work that had saturable inductors not for current regulation for arc stabilisation, they were butters sets, now long gone.
The website for the geyser who built the saturable inductor was AAA welder, sadly it doesnt appear to be there any more.