The idea is to make a 175 V secondary, and connect that in series with the primary, so when have 240 V on the primary, and 175 V on the secondary, when they are in series you get about 415 V. (185 V was a mistake, sorry)
I don't know what gauge of wire you would need. If you work out how large you can fit in the space for the secondary, that would be a good start.
The secondary will have to take the full motor current. The primary will have to take 175/240 of the full motor current. From those figures, and the sizes of the windings, you can work out the resistive losses. They should be less than about 3% of the power.