Three Phase Converter Question

I've got a working 7.5 HP idler running a true 5 HP (24 amp draw) 120
gallon air compressor plus my Bridgeport and a couple of other smaller
motors. This motor runs on a 40 amp breaker out of a 200 amp main. I
need to increase my service to accommodate a three phase welder. Is it
possible to add another motor in the circuit rather than build a new
larger system from scratch. My idea is to break the circuit between the
existing 7.5 HP idler and the air compressor/ mill and drop each of the
three legs down to a disconnect. Then from the disconnect wire the
three legs into a larger motor which would feed the welder when needed
but would be dropped from the circuit when not using the welder. The
welder requires about 35 amps line input and outputs 250 amps at 100%
duty cycle.
Anyone with experience with this? I'm not sure this is feasible. I'm
unsure of how big of a 'secondary idler' I'll need and unsure of how
big of a motor the 7.5 HP 'primary idler' will start. What will this do
to my feed from the main to the 7.5 HP motor? Will I need to increase
the wire size and breaker size from the 7.5 HP motor to the 200 Amp
main service, or should I wire the larger motor as the primary Idler
straight from the 200 Amp Main and use the 7.5 to supply the additional
power for the welder?
All comments/suggestions welcome. Best regards
"Suppose you were an idiot. And suppose you were a member of Congress..
But I repeat myself."

--Mark Twain