you misunderstood, I mean I only use the split-phase hookup for welding, I never powered any equipment off of it other then a plasma cutter, mig and stick welders.
I want to use the PSU for development of custom equipment, possibly even welders, but I don;'t intend to do anything then testing off of it.
What is the proper tool for developing a plasma cutter, welder, induction heater, plasma supply, motor driver, etc?
The design process I had in mind
1) build control electronics, H-bridge/power controller (maybe not always H-bridge)
2) do load testing on robust lab power supply (what I am looking for)
3) do process testing at reduced load to the limits of the derated supply (its $$$)
4) do testing like noise injection into controller, etc.. all while hooked up to current limited precision lab supply under some load, eventually to max
5) develop AC side of power supply, including protection, filtering, AC-DC conversion for rail, inrush, PFC
5) test AC side alone on "AC Source" unit with loads etc
6) combine AC and DC side on AC source unit. Probobly run at reduced load and do things like LISN tests for known good supply, noise injection if safe, radiated emissions rough in, etc
7) Test unit on line voltage, LISN test, over load testing, etc (whatever can be done, this step is still in development, quite complicated). This is the 'out in the wild test' conducted with the more 'primitive' equipment like VARIAC and/or some kind of saturatable reactor at higher power levels, realistic line impedance, etc. Basically full power above the efficiency of the lab power supply, AC source. Possibly with things like surge-loading testing by peak switching, effect of real mains interference/nonidealities.

use unit for whatever workshop stuff
The idea is to be able to use the same good precise work practices as you do with microelectronics but on a power electronics scale up to 300V/10A or so, for 'breadboard' type testing (obviously not on a bread board, meant to be safer, but I mean like power electronics Manhattan prototyping in special test chassis etc.
I don't like the idea of using unregulated voltage or weird electronics for testing purposes that would seem cheap in a microelectronics laboratory (i.e. who that is seriously interested in learning and testing (not just getting it done) would use a unregulated line connected supply for doing bread when they are interested in things like spectrum analyzer measurements?). I want the same care and precision of the microelectronics laboratory in the power electronics laboratory.
The design may not even go into AC side development (other then testing), for instance if an appropriate switching regulated supply is found (like a meanwell type deal). It would likely be safer & more reliable But I still want a adjustable lab supply that can be used for DC side development.
Sorensen has me covered up to around 600W@150V