I have an unused 800W PC power supply.

I would like to know if there is any way to modify it so that it has a single output at 40 or 50V.

I don't know if it is possible to use a single output of the ones that the power supply has or if they can be "combined" into one.

For example, the power supply has 4 12V outputs at 18A as maximum load.

 

IIRC all the power outputs are referenced to the same common.
(GND) or Common.

 

Converting 120Vac to 48Vdc might use that new rectifier, the metal enclosure.
Exactly how many amps at 48Vdc do you need?
An important choice involves the cost of running a big power supply every month
because the latest step down converters can lower your electric bill having very high efficiency.
That is why I started by saying 120VAC to 48VDC It was unheard of earlier that conversion could do that.
I would use a 98% efficient converter and forget that old junk. I might consider an 85%
conversion if the PS is only used once in a while and are there any programs that helps pay for a star rated PS.