Further to tcmtech's question, does it need to be a regulated supply - that is, maintaining exactly 150V whether the load is 0A or 40A - and how much ripple (variation either side of the set voltage at the line or switching frequency) is acceptible?
If neither of these things are critical then a variable transformer plus heavy rectifier and massive caps would certainly be the simplest and most reliable, albeit an expensive and massive system.
I take it size and weight are not an issue?
If you're lucky you might be able to find secondhand a DC welder as a basis.
Please do elaborate on what you're planning to use it for though!