Transformer for Power Supply

I am modifying my Emco F3 mill. The current project is to add a stepper and ballscrew to the Y Axis, as the first step of a conversion to 3 axis CNC. The mechanical aspects are coming along nicely.

The stepper requires up to 70V DC at 4A (sized using the excellent stepper sizing spreadsheet on this board).

Mains power to the machine is via a 3 phase (no neutral) supply at ~400V (NZ domestic 3 phase specification).

My understanding is that the 400V is the line voltage i.e. the voltage between any two of the 3 supply lines. As the supply does not have a neutral line I cannot make a connection between one of the supply lines and neutral to feed a standard 230VAC (primary) transformer.

I also understand that as the secondary output from a transformer is AC, I will need to make an RMS adjustment so I should be adjusting 70VDC by the RMS factor (1.4) which is approx 50VAC secondary. Obviously this will need to be calculated to account for rectification and other losses (later).

So I think I need something like a 400/50V transformer with a 300VA rating - Am I on the right track ?

Should I be using a 3 phase transformer: connecting the primary as a delta, and the secondary as a star, essentially building a triple rectifier/smoothing circuit for each of the three (eventual) steppers.

And finally, could I use a suitably rated SMPS (400V input) instead of building my own ?