Just adding my 2 cents since donw sent a PM asking me to comment on 'zig-zag' transformers.
Zig-zag transformers are autotransformer arrangements used to derive a neutral for a balanced 3 phase source. These are used, for example, to ground a previously ungrounded delta source. The combination of a delta source and a zig-zag derived neutral is very similar to a true wye source. If a zig-zag transformer is properly sized, it can be used to feed line-neutral loads.
As has already been noted, a zig-zag transformer could not possibly be used in this situation, because it would convert the 240V secondary into a 240/139V secondary source, and the OP requires a 120/240V single phase source.
The OP has also brought up the possibility of a single phase derived neutral. A 240V coil with a center tap can be used as the single phase 'grounding transformer', to derive a neutral. However it would require significant engineering to properly size this transformer. The cost of this engineering is probable significantly greater than the value of a 75kVA transformer. As gar says, the circuit in post 26 would work in theory; I simply don't think it is of practical benefit after you consider the required design engineering.
Post #2 incorrectly assumes a single phase transformer, but offers an interesting possibility.
This transformer happens to be conveniently located, no longer being used for 240V loads, and oversized for the new application. I believe that you could disconnect one of the legs, and use it as a single phase 960V to 480V single phase transformer. You could then use this as 480V to 240V single phase transformer with a center tapped secondary. The resulting capacity would only be 25kVA, but you could wire and protect it normally as a 25 kVA transformer without any sort of fancy wiring or calculations.
-Jon