Ok, it is far more clear now.
Of course, it is a lot of power and you can't solve your problem very cheap.
It is necessary to choose an isolated output....Even so, 300V is a dangerous voltage, I hope you know what you are doing and that you are aware of the dangers of electricity.
First, look for an used isolation transformer 240/240 V of 3 or 4 KVA's....As you use it only 40 minutes, it's enough as power ratings.
Look also for an used transformer 240/48V 800VA.
Connect the secondaries of the two transformers in serie so you add the voltages , and a 40A bridge rectifier.
No need of condensators.
The primary of the 240/240V transformer is directly cnnected to the main's voltage (240V)
The primary of the 240/48V transformer should be controlled by a variac 1KVA 0-240V.
Don't forget to protect the primaries of the transformers by fuses.
Do not use a not isolated power supply, it is far too dangerous.....
NB: if current is not enough, you could add another transformer 240/48V 800VA in serie, with primary directly connected to the mains.
NB2: if you use forced cooling of the transormers (not true for the variac), you can use tranformers of lower power ratings down to max 1/2 the specified power ratings.
NB3: all this at your own risks.....