3 Phase Voltage Imbalance
If your rotary phase converter's output is wired for high-leg delta

then I'll present the following as a possibility.  

When wired correctly, you'd expect to see something like this when measuring from each phase to neutral:

and your measurements would be L1 &amp; L2 = 120V, L3 = 208V.  

However, if your phase converter is internally miswired like this:

then your result could look something like this instead:

and your measurements would be something like L1 &amp; L2 = 120V, L3 = 317V.
So - not the 350V you're seeing, but maybe a possibility?