The 6000 is not like that. After the inlet box the three phases go to the power conditioner which has a 3-phase thyristor controlled rectifier. From there its 300v up to the regulators. I remember my 11/780 was like you described.
Oh, that shouldn't be too bad then: Most good networking power supplies step the 120/208/240 input up to a 300v interstage voltage before feeding the various supplies (48v, 12v, etc). In fact I have taken Cisco 6009 power supplies apart to get the juicy 300v for building a quick and dirty pack charger for my truck (you can mod the supply to deliver up to 390v constant voltage instead of 300v).
So get a few of these, have them power the 300v rail and you're in business. The older Vaxes had linear power supplies if I recall up to the time of the Decsystem/2020 (when they went switching at last)
C
